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[PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

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  [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-08-30 21:10 +0200
    [PATCH 2/2] mm/slub: do not bypass memcg reclaim for high-order page allocation Vladimir Davydov <vdavydov@parallels.com> - 2015-08-30 21:10 +0200
    [PATCH 1/2] mm/slab: skip memcg reclaim only if in atomic context Vladimir Davydov <vdavydov@parallels.com> - 2015-08-30 21:10 +0200
    Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Michal Hocko <mhocko@kernel.org> - 2015-08-31 15:30 +0200
      Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Tejun Heo <tj@kernel.org> - 2015-08-31 15:50 +0200
        Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Tejun Heo <tj@kernel.org> - 2015-08-31 16:40 +0200
          Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-08-31 17:20 +0200
            Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Tejun Heo <tj@kernel.org> - 2015-08-31 17:50 +0200
              Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-08-31 19:00 +0200
                Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Tejun Heo <tj@kernel.org> - 2015-08-31 19:10 +0200
                  Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-08-31 21:30 +0200
                    Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Christoph Lameter <cl@linux.com> - 2015-08-31 22:30 +0200
                      Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-09-01 11:30 +0200
        Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-08-31 16:40 +0200
      Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-08-31 16:30 +0200
        Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Tejun Heo <tj@kernel.org> - 2015-08-31 16:50 +0200
          Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-08-31 17:30 +0200
        Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Michal Hocko <mhocko@kernel.org> - 2015-09-01 14:40 +0200
          Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-09-01 15:50 +0200
            Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Michal Hocko <mhocko@kernel.org> - 2015-09-01 17:10 +0200
              Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-09-01 19:00 +0200
                Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Michal Hocko <mhocko@kernel.org> - 2015-09-01 20:40 +0200
                  Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-09-02 11:40 +0200
                    Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Christoph Lameter <cl@linux.com> - 2015-09-02 20:20 +0200
                      Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-09-03 11:40 +0200
                    Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Tejun Heo <tj@kernel.org> - 2015-09-03 18:40 +0200
                      Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-09-04 13:20 +0200
                        Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Tejun Heo <tj@kernel.org> - 2015-09-04 17:50 +0200
                          Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Vladimir Davydov <vdavydov@parallels.com> - 2015-09-04 20:30 +0200
                            Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Tejun Heo <tj@kernel.org> - 2015-09-04 21:40 +0200
                    Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is  enabled Michal Hocko <mhocko@kernel.org> - 2015-09-04 16:40 +0200

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#1216966 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

On Tue, Sep 01, 2015 at 05:01:20PM +0200, Michal Hocko wrote:
> On Tue 01-09-15 16:40:03, Vladimir Davydov wrote:
> > On Tue, Sep 01, 2015 at 02:36:12PM +0200, Michal Hocko wrote:
> > > On Mon 31-08-15 17:20:49, Vladimir Davydov wrote:
> {...}
> > > >  1. SLAB. Suppose someone calls kmalloc_node and there is enough free
> > > >     memory on the preferred node. W/o memcg limit set, the allocation
> > > >     will happen from the preferred node, which is OK. If there is memcg
> > > >     limit, we can currently fail to allocate from the preferred node if
> > > >     we are near the limit. We issue memcg reclaim and go to fallback
> > > >     alloc then, which will most probably allocate from a different node,
> > > >     although there is no reason for that. This is a bug.
> > > 
> > > I am not familiar with the SLAB internals much but how is it different
> > > from the global case. If the preferred node is full then __GFP_THISNODE
> > > request will make it fail early even without giving GFP_NOWAIT
> > > additional access to atomic memory reserves. The fact that memcg case
> > > fails earlier is perfectly expected because the restriction is tighter
> > > than the global case.
> > 
> > memcg restrictions are orthogonal to NUMA: failing an allocation from a
> > particular node does not mean failing memcg charge and vice versa.
> 
> Sure memcg doesn't care about NUMA it just puts an additional constrain
> on top of all existing ones. The point I've tried to make is that the
> logic is currently same whether it is page allocator (with the node
> restriction) or memcg (cumulative amount restriction) are behaving
> consistently. Neither of them try to reclaim in order to achieve its
> goals. How conservative is memcg about allowing GFP_NOWAIT allocation
> is a separate issue and all those details belong to memcg proper same
> as the allocation strategy for these allocations belongs to the page
> allocator.
>  
> > > How the fallback is implemented and whether trying other node before
> > > reclaiming from the preferred one is reasonable I dunno. This is for
> > > SLAB to decide. But ignoring GFP_NOWAIT for this path makes the behavior
> > > for memcg enabled setups subtly different. And that is bad.
> > 
> > Quite the contrary. Trying to charge memcg w/o __GFP_WAIT while
> > inspecting if a NUMA node has free pages makes SLAB behaviour subtly
> > differently: SLAB will walk over all NUMA nodes for nothing instead of
> > invoking memcg reclaim once a free page is found.
> 
> So you are saying that the SLAB kmem accounting in this particular path
> is suboptimal because the fallback mode doesn't retry local node with
> the reclaim enabled before falling back to other nodes?

I'm just pointing out some subtle behavior changes in slab you were
opposed to.

> I would consider it quite surprising as well even for the global case
> because __GFP_THISNODE doesn't wake up kswapd to make room on that node.
> 
> > You are talking about memcg/kmem accounting as if it were done in the
> > buddy allocator on top of which the slab layer is built knowing nothing
> > about memcg accounting on the lower layer. That's not true and that
> > simply can't be true. Kmem accounting is implemented at the slab layer.
> > Memcg provides its memcg_charge_slab/uncharge methods solely for
> > slab core, so it's OK to have some calling conventions between them.
> > What we are really obliged to do is to preserve behavior of slab's
> > external API, i.e. kmalloc and friends.
> 
> I guess I understand what you are saying here but it sounds like special
> casing which tries to be clever because the current code understands
> both the lower level allocator and kmem charge paths to decide how to

What do you mean by saying "it understands the lower level allocator"?
AFAIK we have memcg callbacks only in special places, like page fault
handler or kmalloc.

> juggle with them. This is imho bad and hard to maintain long term.

We already juggle. Just grep where and how we insert
mem_cgroup_try_charge.

> 
> > > >  2. SLUB. Someone calls kmalloc and there is enough free high order
> > > >     pages. If there is no memcg limit, we will allocate a high order
> > > >     slab page, which is in accordance with SLUB internal logic. With
> > > >     memcg limit set, we are likely to fail to charge high order page
> > > >     (because we currently try to charge high order pages w/o __GFP_WAIT)
> > > >     and fallback on a low order page. The latter is unexpected and
> > > >     unjustified.
> > > 
> > > And this case very similar and I even argue that it shows more
> > > brokenness with your patch. The SLUB allocator has _explicitly_ asked
> > > for an allocation _without_ reclaim because that would be unnecessarily
> > > too costly and there is other less expensive fallback. But memcg would
> > 
> > You are ignoring the fact that, in contrast to alloc_pages, for memcg
> > there is practically no difference between charging a 4-order page or a
> > 1-order page.
> 
> But this is an implementation details which might change anytime in
> future.

The fact that memcg reclaim does not invoke compactor is indeed an
implementation detail, but how can it change?

> 
> > OTOH, using 1-order pages where we could go with 4-order
> > pages increases page fragmentation at the global level. This subtly
> > breaks internal SLUB optimization. Once again, kmem accounting is not
> > something staying aside from slab core, it's a part of slab core.
> 
> This is certainly true and it is what you get when you put an additional
> constrain on top of an existing one. You simply cannot get both the
> great performance _and_ a local memory restriction.

So what? We shouldn't even try?

> 
> > > be ignoring this with your patch AFAIU and break the optimization. There
> > > are other cases like that. E.g. THP pages are allocated without GFP_WAIT
> > > when defrag is disabled.
> > 
> > It might be wrong. If we can't find a continuous 2Mb page, we should
> > probably give up instead of calling compactor. For memcg it might be
> > better to reclaim some space for 2Mb page right now and map a 2Mb page
> > instead of reclaiming space for 512 4Kb pages a moment later, because in
> > memcg case there is absolutely no difference between reclaiming 2Mb for
> > a huge page and 2Mb for 512 4Kb pages.
> 
> Or maybe the whole reclaim just doesn't pay off because the TLB savings
> will never compensate for the reclaim. The defrag knob basically says
> that we shouldn't try to opportunistically prepare a room for the THP
> page.

And why is it called "defrag" then?

> 
> > > > That being said, this is the fix at the right layer.
> > > > 
> > > > > Either we should start failing GFP_NOWAIT charges when we are above
> > > > > high wmark or deploy an additional catchup mechanism as suggested by
> > > > > Tejun.
> > > > 
> > > > The mechanism proposed by Tejun won't help us to avoid allocation
> > > > failures if we are hitting memory.max w/o __GFP_WAIT or __GFP_FS.
> > > 
> > > Why would be that a problem. The _hard_ limit is reached and reclaim
> > > cannot make any progress. An allocation failure is to be expected.
> > > GFP_NOWAIT will fail normally and GFP_NOFS will attempt to reclaim
> > > before failing.
> > 
> > Quoting my e-mail to Tejun explaining why using task_work won't help if
> > we don't fix SLAB/SLUB:
> > 
> > : Generally speaking, handing over reclaim responsibility to task_work
> > : won't help, because there might be cases when a process spends quite a
> > : lot of time in kernel invoking lots of GFP_KERNEL allocations before
> > : returning to userspace. Without fixing slab/slub, such a process will
> > : charge w/o __GFP_WAIT and therefore can exceed memory.high and reach
> > : memory.max. If there are no other active processes in the cgroup, the
> > : cgroup can stay with memory.high excess for a relatively long time
> > : (suppose the process was throttled in kernel), possibly hurting the rest
> > : of the system. What is worse, if the process happens to invoke a real
> > : GFP_NOWAIT allocation when it's about to hit the limit, it will fail.
> > 
> > For a kmalloc user that's completely unexpected.
> 
> We have the global reclaim which handles the global memory pressure. And
> until the hard limit is enforced I do not see what is the huge problem
> here. Sure we can have high limit in excess but that is to be expected.

What exactly is to be expected? Is it OK if memory.high is just ignored?

> Same as failing allocations for the hard limit enforcement.

If a kmem allocation fails, your app is likely to fail too. Nobody
expects write/read fail with ENOMEM if there seems to be enough
reclaimable memory. If we try to fix the GFP_NOWAIT problem only by
using task_work reclaim, it won't be a complete fix, because a failure
may still occur as I described above.

> 
> Maybe moving whole high limit reclaim to the delayed context is not what
> we will end up with and reduce this only for GFP_NOWAIT or other weak
> reclaim contexts. This is to be discussed of course.

Yeah, but w/o fixing kmalloc it may happen that *every* allocation will
be GFP_NOWAIT. It'd complicate the implementation.

> 
> > > > To fix GFP_NOFS/GFP_NOWAIT failures we just need to start reclaim when
> > > > the gap between limit and usage is getting too small. It may be done
> > > > from a workqueue or from task_work, but currently I don't see any reason
> > > > why complicate and not just start reclaim directly, just like
> > > > memory.high does.
> > > 
> > > Yes we can do better than we do right now. But that doesn't mean we
> > > should put hacks all over the place and lie about the allocation
> > > context.
> > 
> > What do you mean by saying "all over the place"? It's a fix for kmem
> > implementation, to be more exact for the part of it residing in the slab
> > core.
> 
> I meant into two slab allocators currently because of the implementation
> details which are spread into three different places - page allocator,
> memcg charging code and the respective slab allocator specific details.

If we remove kmem accounting, we will still have implementation details
spread over page allocator, reclaimer, rmap, memcg. Slab is not the
worst part of it IMO. Anyway, kmem accounting can't be implemented
solely in memcg.

> 
> > Everyone else, except a couple of kmem users issuing alloc_page
> > directly like threadinfo, will use kmalloc and know nothing what's going
> > on there and how all this accounting stuff is handled - they will just
> > use plain old convenient kmalloc, which works exactly as it does in the
> > root cgroup.
> 
> If we ever grow more users and charge more kernel memory then they might
> be doing similar assumptions and tweak allocation/charge context and we
> would end up in a bigger mess. It makes much more sense to have
> allocation and charge context consistent.

What new users? Why can't they just call kmalloc?

Thanks,
Vladimir
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#1217019 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

On Tue 01-09-15 19:55:54, Vladimir Davydov wrote:
> On Tue, Sep 01, 2015 at 05:01:20PM +0200, Michal Hocko wrote:
> > On Tue 01-09-15 16:40:03, Vladimir Davydov wrote:
> > > On Tue, Sep 01, 2015 at 02:36:12PM +0200, Michal Hocko wrote:
[...]
> > > > How the fallback is implemented and whether trying other node before
> > > > reclaiming from the preferred one is reasonable I dunno. This is for
> > > > SLAB to decide. But ignoring GFP_NOWAIT for this path makes the behavior
> > > > for memcg enabled setups subtly different. And that is bad.
> > > 
> > > Quite the contrary. Trying to charge memcg w/o __GFP_WAIT while
> > > inspecting if a NUMA node has free pages makes SLAB behaviour subtly
> > > differently: SLAB will walk over all NUMA nodes for nothing instead of
> > > invoking memcg reclaim once a free page is found.
> > 
> > So you are saying that the SLAB kmem accounting in this particular path
> > is suboptimal because the fallback mode doesn't retry local node with
> > the reclaim enabled before falling back to other nodes?
> 
> I'm just pointing out some subtle behavior changes in slab you were
> opposed to.

I guess we are still not at the same page here. If the slab has a subtle
behavior (and from what you are saying it seems it has the same behavior
at the global scope) then we should strive to fix it rather than making
it more obscure just to not expose GFP_NOWAIT to memcg which is not
handled properly currently wrt. high limit (more on that below) which
was the primary motivation for the patch AFAIU.

> > I would consider it quite surprising as well even for the global case
> > because __GFP_THISNODE doesn't wake up kswapd to make room on that node.
> > 
> > > You are talking about memcg/kmem accounting as if it were done in the
> > > buddy allocator on top of which the slab layer is built knowing nothing
> > > about memcg accounting on the lower layer. That's not true and that
> > > simply can't be true. Kmem accounting is implemented at the slab layer.
> > > Memcg provides its memcg_charge_slab/uncharge methods solely for
> > > slab core, so it's OK to have some calling conventions between them.
> > > What we are really obliged to do is to preserve behavior of slab's
> > > external API, i.e. kmalloc and friends.
> > 
> > I guess I understand what you are saying here but it sounds like special
> > casing which tries to be clever because the current code understands
> > both the lower level allocator and kmem charge paths to decide how to
> 
> What do you mean by saying "it understands the lower level allocator"?

I mean it requires/abuses special behavior from the page allocator like
__GFP_THISNODE && !wait for the hot path. 

> AFAIK we have memcg callbacks only in special places, like page fault
> handler or kmalloc.

But anybody might opt-in to be charged. I can see some other buffers
which are even not accounted for right now will be charged in future.

> > juggle with them. This is imho bad and hard to maintain long term.
> 
> We already juggle. Just grep where and how we insert
> mem_cgroup_try_charge.

We should always preserve the gfp context (at least its reclaim
part). If we are not then it is a bug.
 
> > > > >  2. SLUB. Someone calls kmalloc and there is enough free high order
> > > > >     pages. If there is no memcg limit, we will allocate a high order
> > > > >     slab page, which is in accordance with SLUB internal logic. With
> > > > >     memcg limit set, we are likely to fail to charge high order page
> > > > >     (because we currently try to charge high order pages w/o __GFP_WAIT)
> > > > >     and fallback on a low order page. The latter is unexpected and
> > > > >     unjustified.
> > > > 
> > > > And this case very similar and I even argue that it shows more
> > > > brokenness with your patch. The SLUB allocator has _explicitly_ asked
> > > > for an allocation _without_ reclaim because that would be unnecessarily
> > > > too costly and there is other less expensive fallback. But memcg would
> > > 
> > > You are ignoring the fact that, in contrast to alloc_pages, for memcg
> > > there is practically no difference between charging a 4-order page or a
> > > 1-order page.
> > 
> > But this is an implementation details which might change anytime in
> > future.
> 
> The fact that memcg reclaim does not invoke compactor is indeed an
> implementation detail, but how can it change?

Compaction is indeed not something memcg reclaim cares about right now
or will care in foreseeable future. I meant something else. order-1 vs.
ordern-N differ in the reclaim target which then controls the potential
latency of the reclaim. The fact that order-1 and order-4 do not really
make any difference _right now_ because of the large SWAP_CLUSTER_MAX is
the implementation detail I was referring to.
 
> > > OTOH, using 1-order pages where we could go with 4-order
> > > pages increases page fragmentation at the global level. This subtly
> > > breaks internal SLUB optimization. Once again, kmem accounting is not
> > > something staying aside from slab core, it's a part of slab core.
> > 
> > This is certainly true and it is what you get when you put an additional
> > constrain on top of an existing one. You simply cannot get both the
> > great performance _and_ a local memory restriction.
> 
> So what? We shouldn't even try?

Of course you can try. Then the question is what are costs/benefits
(both performance and maintainability). I didn't say those two patches
are incorrect (the original kmalloc gfp mask is obeyed).
They just seem targeting a wrong layer IMO. Alternative solutions were
not attempted and measured for typical workloads. If we find out that
addressing GFP_NOWAIT at memcg level will be viable for most reasonable
loads and corner cases are at least not causing runaways which would be
hard to address then let's put workarounds where they are necessary.

> > > > be ignoring this with your patch AFAIU and break the optimization. There
> > > > are other cases like that. E.g. THP pages are allocated without GFP_WAIT
> > > > when defrag is disabled.
> > > 
> > > It might be wrong. If we can't find a continuous 2Mb page, we should
> > > probably give up instead of calling compactor. For memcg it might be
> > > better to reclaim some space for 2Mb page right now and map a 2Mb page
> > > instead of reclaiming space for 512 4Kb pages a moment later, because in
> > > memcg case there is absolutely no difference between reclaiming 2Mb for
> > > a huge page and 2Mb for 512 4Kb pages.
> > 
> > Or maybe the whole reclaim just doesn't pay off because the TLB savings
> > will never compensate for the reclaim. The defrag knob basically says
> > that we shouldn't try to opportunistically prepare a room for the THP
> > page.
> 
> And why is it called "defrag" then?

Do not ask me about the naming. If this was only about compaction then
the allocator might be told about that by a special GFP flag. The memcg
could be in line with that. But the point remains. If the defrag is
a knob to make the page fault THP path lighter then no memcg reclaim is a
reasonable to do.
 
[...]
> > > Quoting my e-mail to Tejun explaining why using task_work won't help if
> > > we don't fix SLAB/SLUB:
> > > 
> > > : Generally speaking, handing over reclaim responsibility to task_work
> > > : won't help, because there might be cases when a process spends quite a
> > > : lot of time in kernel invoking lots of GFP_KERNEL allocations before
> > > : returning to userspace. Without fixing slab/slub, such a process will
> > > : charge w/o __GFP_WAIT and therefore can exceed memory.high and reach
> > > : memory.max. If there are no other active processes in the cgroup, the
> > > : cgroup can stay with memory.high excess for a relatively long time
> > > : (suppose the process was throttled in kernel), possibly hurting the rest
> > > : of the system. What is worse, if the process happens to invoke a real
> > > : GFP_NOWAIT allocation when it's about to hit the limit, it will fail.
> > > 
> > > For a kmalloc user that's completely unexpected.
> > 
> > We have the global reclaim which handles the global memory pressure. And
> > until the hard limit is enforced I do not see what is the huge problem
> > here. Sure we can have high limit in excess but that is to be expected.
> 
> What exactly is to be expected? Is it OK if memory.high is just ignored?

It is not OK to be ignored altogether. The high limit is where the
throttling should start. And we currently do not handle GFP_NOWAIT which
is something to be solved. We shouldn't remove GFP_NOWAIT callers as a
workaround.

There are more things to do here. We can perform the reclaim from the
delayed context where the direct reclaim is not allowed/requested. And
we can start failing GFP_NOWAIT on an excessive high limit breach when
the delayed reclaim doesn't catch up with the demand. This is basically
what we do on the global level.
If even this is not sufficient and the kernel allows for a lot of
allocations in the single run, which would be something to look at in
the first place, then we have global mechanisms to mitigate that.

memory.high is an opportunistic memory isolation. It doesn't guarantee a
complete isolation. The hard limit is for that purpose.

> > Same as failing allocations for the hard limit enforcement.
> 
> If a kmem allocation fails, your app is likely to fail too. Nobody
> expects write/read fail with ENOMEM if there seems to be enough
> reclaimable memory. If we try to fix the GFP_NOWAIT problem only by
> using task_work reclaim, it won't be a complete fix, because a failure
> may still occur as I described above.

You cannot have a system which cannot tolerate failures and require
memory restrictions. These two requirements simply go against each other.
Moreover GPF_NOWAIT context is really light and should always have a
fallback mode otherwise you get what you are saying - failures with
reclaimable memory. And this is very much the case for the global case
as well.

> > Maybe moving whole high limit reclaim to the delayed context is not what
> > we will end up with and reduce this only for GFP_NOWAIT or other weak
> > reclaim contexts. This is to be discussed of course.
> 
> Yeah, but w/o fixing kmalloc it may happen that *every* allocation will
> be GFP_NOWAIT. It'd complicate the implementation.

OK, but that is the case for the global case already. MM resp. memcg has
to say at when to stop it. The global case handles that at the page
allocator layer and memcg should do something similar at the charge
level.

[...]
> > > What do you mean by saying "all over the place"? It's a fix for kmem
> > > implementation, to be more exact for the part of it residing in the slab
> > > core.
> > 
> > I meant into two slab allocators currently because of the implementation
> > details which are spread into three different places - page allocator,
> > memcg charging code and the respective slab allocator specific details.
> 
> If we remove kmem accounting, we will still have implementation details
> spread over page allocator, reclaimer, rmap, memcg. Slab is not the
> worst part of it IMO. Anyway, kmem accounting can't be implemented
> solely in memcg.

The current state is quite complex already and making it even more
complex by making allocation and charge context inconsistent is not really
desirable.
 
> > > Everyone else, except a couple of kmem users issuing alloc_page
> > > directly like threadinfo, will use kmalloc and know nothing what's going
> > > on there and how all this accounting stuff is handled - they will just
> > > use plain old convenient kmalloc, which works exactly as it does in the
> > > root cgroup.
> > 
> > If we ever grow more users and charge more kernel memory then they might
> > be doing similar assumptions and tweak allocation/charge context and we
> > would end up in a bigger mess. It makes much more sense to have
> > allocation and charge context consistent.
> 
> What new users? Why can't they just call kmalloc?

What about direct users of the page allocator. Why should they pay cost
for more complex/expensive code paths when they do not need sub-page
sizes.

-- 
Michal Hocko
SUSE Labs
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#1217454 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

[
  I'll try to summarize my point in one hunk instead of spreading it all
  over the e-mail, because IMO it's becoming a kind of difficult to
  follow. If you think that there's a question I dodge, please let me
  now and I'll try to address it separately.

  Also, adding Johannes to Cc (I noticed that I accidentally left him
  out), because this discussion seems to be fundamental and may affect
  our further steps dramatically.
]

On Tue, Sep 01, 2015 at 08:38:50PM +0200, Michal Hocko wrote:
> On Tue 01-09-15 19:55:54, Vladimir Davydov wrote:
> > On Tue, Sep 01, 2015 at 05:01:20PM +0200, Michal Hocko wrote:
> > > On Tue 01-09-15 16:40:03, Vladimir Davydov wrote:
> > > > On Tue, Sep 01, 2015 at 02:36:12PM +0200, Michal Hocko wrote:
> [...]
> > > > > How the fallback is implemented and whether trying other node before
> > > > > reclaiming from the preferred one is reasonable I dunno. This is for
> > > > > SLAB to decide. But ignoring GFP_NOWAIT for this path makes the behavior
> > > > > for memcg enabled setups subtly different. And that is bad.
> > > > 
> > > > Quite the contrary. Trying to charge memcg w/o __GFP_WAIT while
> > > > inspecting if a NUMA node has free pages makes SLAB behaviour subtly
> > > > differently: SLAB will walk over all NUMA nodes for nothing instead of
> > > > invoking memcg reclaim once a free page is found.
> > > 
> > > So you are saying that the SLAB kmem accounting in this particular path
> > > is suboptimal because the fallback mode doesn't retry local node with
> > > the reclaim enabled before falling back to other nodes?
> > 
> > I'm just pointing out some subtle behavior changes in slab you were
> > opposed to.
> 
> I guess we are still not at the same page here. If the slab has a subtle
> behavior (and from what you are saying it seems it has the same behavior
> at the global scope) then we should strive to fix it rather than making
> it more obscure just to not expose GFP_NOWAIT to memcg which is not
> handled properly currently wrt. high limit (more on that below) which
> was the primary motivation for the patch AFAIU.

Slab is a kind of abnormal alloc_pages user. By calling alloc_pages_node
with __GFP_THISNODE and w/o __GFP_WAIT before falling back to
alloc_pages with the caller's context, it does the job normally done by
alloc_pages itself. It's not what is done massively.

Leaving slab charge path as is looks really ugly to me. Look, slab
iterates over all nodes, inspecting if they have free pages and fails
even if they do due to the memcg constraint...

My point is that what slab does is a pretty low level thing, normal
users call alloc_pages or kmalloc with flags corresponding to their
context. Of course, there may be special users trying optimistically
GFP_NOWAIT, but they aren't massive, and that simplifies things for
memcg a lot. I mean if we can rely on the fact that the number of
GFP_NOWAIT allocations that can occur in a row is limited we can use
direct reclaim (like memory.high) and/or task_work reclaim to fix
GFP_NOWAIT failures. Otherwise, we have to mimic the global alloc with
most its heuristics. I don't think that copying those heuristics is the
right thing to do, because in memcg case the same problems may be
resolved much easier, because we don't actually experience real memory
shortage when hitting the limit.

Moreover, we already treat some flags not in the same way as in case of
slab for simplicity. E.g. we let __GFP_NOFAIL allocations go uncharged
instead of retrying infinitely. We ignore __GFP_THISNODE thing and we
just cannot take it into account. We ignore allocation order, because
that makes no sense for memcg.

To sum it up. Basically, there are two ways of handling kmemcg charges:

 1. Make the memcg try_charge mimic alloc_pages behavior.
 2. Make API functions (kmalloc, etc) work in memcg as if they were
    called from the root cgroup, while keeping interactions between the
    low level subsys (slab) and memcg private.

Way 1 might look appealing at the first glance, but at the same time it
is much more complex, because alloc_pages has grown over the years to
handle a lot of subtle situations that may arise on global memory
pressure, but impossible in memcg. What does way 1 give us then? We
can't insert try_charge directly to alloc_pages and have to spread its
calls all over the code anyway, so why is it better? Easier to use it in
places where users depend on buddy allocator peculiarities? There are
not many such users.

I understand that the idea of way 1 is to provide a well-defined memcg
API independent of the rest of the code, but that's just impossible. You
need special casing anyway. E.g. you need those get/put_kmem_cache
helpers, which exist solely for SLAB/SLUB. You need all this special
stuff for growing per-memcg array in list_lru and kmem_cache, which
exists solely for memcg-vs-list_lru and memcg-vs-slab interactions. We
even handle kmem_cache destruction on memcg offline differently for SLAB
and SLUB for performance reasons.

Way 2 gives us more space to maneuver IMO. SLAB/SLUB may do weird tricks
for optimization, but their API is well defined, so we just make kmalloc
work as expected while providing inter-subsys calls, like
memcg_charge_slab, for SLAB/SLUB that have their own conventions. You
mentioned kmem users that allocate memory using alloc_pages. There is an
API function for them too, alloc_kmem_pages. Everything behind the API
is hidden and may be done in such a way to achieve optimal performance.

Thanks,
Vladimir

> 
> > > I would consider it quite surprising as well even for the global case
> > > because __GFP_THISNODE doesn't wake up kswapd to make room on that node.
> > > 
> > > > You are talking about memcg/kmem accounting as if it were done in the > > > > buddy allocator on top of which the slab layer is built knowing nothing
> > > > about memcg accounting on the lower layer. That's not true and that
> > > > simply can't be true. Kmem accounting is implemented at the slab layer.
> > > > Memcg provides its memcg_charge_slab/uncharge methods solely for
> > > > slab core, so it's OK to have some calling conventions between them.
> > > > What we are really obliged to do is to preserve behavior of slab's
> > > > external API, i.e. kmalloc and friends.
> > > 
> > > I guess I understand what you are saying here but it sounds like special
> > > casing which tries to be clever because the current code understands
> > > both the lower level allocator and kmem charge paths to decide how to
> > 
> > What do you mean by saying "it understands the lower level allocator"?
> 
> I mean it requires/abuses special behavior from the page allocator like
> __GFP_THISNODE && !wait for the hot path. 
> 
> > AFAIK we have memcg callbacks only in special places, like page fault
> > handler or kmalloc.
> 
> But anybody might opt-in to be charged. I can see some other buffers
> which are even not accounted for right now will be charged in future.
> 
> > > juggle with them. This is imho bad and hard to maintain long term.
> > 
> > We already juggle. Just grep where and how we insert
> > mem_cgroup_try_charge.
> 
> We should always preserve the gfp context (at least its reclaim
> part). If we are not then it is a bug.
>  
> > > > > >  2. SLUB. Someone calls kmalloc and there is enough free high order
> > > > > >     pages. If there is no memcg limit, we will allocate a high order
> > > > > >     slab page, which is in accordance with SLUB internal logic. With
> > > > > >     memcg limit set, we are likely to fail to charge high order page
> > > > > >     (because we currently try to charge high order pages w/o __GFP_WAIT)
> > > > > >     and fallback on a low order page. The latter is unexpected and
> > > > > >     unjustified.
> > > > > 
> > > > > And this case very similar and I even argue that it shows more
> > > > > brokenness with your patch. The SLUB allocator has _explicitly_ asked
> > > > > for an allocation _without_ reclaim because that would be unnecessarily
> > > > > too costly and there is other less expensive fallback. But memcg would
> > > > 
> > > > You are ignoring the fact that, in contrast to alloc_pages, for memcg
> > > > there is practically no difference between charging a 4-order page or a
> > > > 1-order page.
> > > 
> > > But this is an implementation details which might change anytime in
> > > future.
> > 
> > The fact that memcg reclaim does not invoke compactor is indeed an
> > implementation detail, but how can it change?
> 
> Compaction is indeed not something memcg reclaim cares about right now
> or will care in foreseeable future. I meant something else. order-1 vs.
> ordern-N differ in the reclaim target which then controls the potential
> latency of the reclaim. The fact that order-1 and order-4 do not really
> make any difference _right now_ because of the large SWAP_CLUSTER_MAX is
> the implementation detail I was referring to.
>  
> > > > OTOH, using 1-order pages where we could go with 4-order
> > > > pages increases page fragmentation at the global level. This subtly
> > > > breaks internal SLUB optimization. Once again, kmem accounting is not
> > > > something staying aside from slab core, it's a part of slab core.
> > > 
> > > This is certainly true and it is what you get when you put an additional
> > > constrain on top of an existing one. You simply cannot get both the
> > > great performance _and_ a local memory restriction.
> > 
> > So what? We shouldn't even try?
> 
> Of course you can try. Then the question is what are costs/benefits
> (both performance and maintainability). I didn't say those two patches
> are incorrect (the original kmalloc gfp mask is obeyed).
> They just seem targeting a wrong layer IMO. Alternative solutions were
> not attempted and measured for typical workloads. If we find out that
> addressing GFP_NOWAIT at memcg level will be viable for most reasonable
> loads and corner cases are at least not causing runaways which would be
> hard to address then let's put workarounds where they are necessary.
> 
> > > > > be ignoring this with your patch AFAIU and break the optimization. There
> > > > > are other cases like that. E.g. THP pages are allocated without GFP_WAIT
> > > > > when defrag is disabled.
> > > > 
> > > > It might be wrong. If we can't find a continuous 2Mb page, we should
> > > > probably give up instead of calling compactor. For memcg it might be
> > > > better to reclaim some space for 2Mb page right now and map a 2Mb page
> > > > instead of reclaiming space for 512 4Kb pages a moment later, because in
> > > > memcg case there is absolutely no difference between reclaiming 2Mb for
> > > > a huge page and 2Mb for 512 4Kb pages.
> > > 
> > > Or maybe the whole reclaim just doesn't pay off because the TLB savings
> > > will never compensate for the reclaim. The defrag knob basically says
> > > that we shouldn't try to opportunistically prepare a room for the THP
> > > page.
> > 
> > And why is it called "defrag" then?
> 
> Do not ask me about the naming. If this was only about compaction then
> the allocator might be told about that by a special GFP flag. The memcg
> could be in line with that. But the point remains. If the defrag is
> a knob to make the page fault THP path lighter then no memcg reclaim is a
> reasonable to do.
>  
> [...]
> > > > Quoting my e-mail to Tejun explaining why using task_work won't help if
> > > > we don't fix SLAB/SLUB:
> > > > 
> > > > : Generally speaking, handing over reclaim responsibility to task_work
> > > > : won't help, because there might be cases when a process spends quite a
> > > > : lot of time in kernel invoking lots of GFP_KERNEL allocations before
> > > > : returning to userspace. Without fixing slab/slub, such a process will
> > > > : charge w/o __GFP_WAIT and therefore can exceed memory.high and reach
> > > > : memory.max. If there are no other active processes in the cgroup, the
> > > > : cgroup can stay with memory.high excess for a relatively long time
> > > > : (suppose the process was throttled in kernel), possibly hurting the rest
> > > > : of the system. What is worse, if the process happens to invoke a real
> > > > : GFP_NOWAIT allocation when it's about to hit the limit, it will fail.
> > > > 
> > > > For a kmalloc user that's completely unexpected.
> > > 
> > > We have the global reclaim which handles the global memory pressure. And
> > > until the hard limit is enforced I do not see what is the huge problem
> > > here. Sure we can have high limit in excess but that is to be expected.
> > 
> > What exactly is to be expected? Is it OK if memory.high is just ignored?
> 
> It is not OK to be ignored altogether. The high limit is where the
> throttling should start. And we currently do not handle GFP_NOWAIT which
> is something to be solved. We shouldn't remove GFP_NOWAIT callers as a
> workaround.
> 
> There are more things to do here. We can perform the reclaim from the
> delayed context where the direct reclaim is not allowed/requested. And
> we can start failing GFP_NOWAIT on an excessive high limit breach when
> the delayed reclaim doesn't catch up with the demand. This is basically
> what we do on the global level.
> If even this is not sufficient and the kernel allows for a lot of
> allocations in the single run, which would be something to look at in
> the first place, then we have global mechanisms to mitigate that.
> 
> memory.high is an opportunistic memory isolation. It doesn't guarantee a
> complete isolation. The hard limit is for that purpose.
> 
> > > Same as failing allocations for the hard limit enforcement.
> > 
> > If a kmem allocation fails, your app is likely to fail too. Nobody
> > expects write/read fail with ENOMEM if there seems to be enough
> > reclaimable memory. If we try to fix the GFP_NOWAIT problem only by
> > using task_work reclaim, it won't be a complete fix, because a failure
> > may still occur as I described above.
> 
> You cannot have a system which cannot tolerate failures and require
> memory restrictions. These two requirements simply go against each other.
> Moreover GPF_NOWAIT context is really light and should always have a
> fallback mode otherwise you get what you are saying - failures with
> reclaimable memory. And this is very much the case for the global case
> as well.
> 
> > > Maybe moving whole high limit reclaim to the delayed context is not what
> > > we will end up with and reduce this only for GFP_NOWAIT or other weak
> > > reclaim contexts. This is to be discussed of course.
> > 
> > Yeah, but w/o fixing kmalloc it may happen that *every* allocation will
> > be GFP_NOWAIT. It'd complicate the implementation.
> 
> OK, but that is the case for the global case already. MM resp. memcg has
> to say at when to stop it. The global case handles that at the page
> allocator layer and memcg should do something similar at the charge
> level.
> 
> [...]
> > > > What do you mean by saying "all over the place"? It's a fix for kmem
> > > > implementation, to be more exact for the part of it residing in the slab
> > > > core.
> > > 
> > > I meant into two slab allocators currently because of the implementation
> > > details which are spread into three different places - page allocator,
> > > memcg charging code and the respective slab allocator specific details.
> > 
> > If we remove kmem accounting, we will still have implementation details
> > spread over page allocator, reclaimer, rmap, memcg. Slab is not the
> > worst part of it IMO. Anyway, kmem accounting can't be implemented
> > solely in memcg.
> 
> The current state is quite complex already and making it even more
> complex by making allocation and charge context inconsistent is not really
> desirable.
>  
> > > > Everyone else, except a couple of kmem users issuing alloc_page
> > > > directly like threadinfo, will use kmalloc and know nothing what's going
> > > > on there and how all this accounting stuff is handled - they will just
> > > > use plain old convenient kmalloc, which works exactly as it does in the
> > > > root cgroup.
> > > 
> > > If we ever grow more users and charge more kernel memory then they might
> > > be doing similar assumptions and tweak allocation/charge context and we
> > > would end up in a bigger mess. It makes much more sense to have
> > > allocation and charge context consistent.
> > 
> > What new users? Why can't they just call kmalloc?
> 
> What about direct users of the page allocator. Why should they pay cost
> for more complex/expensive code paths when they do not need sub-page
> sizes.
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#1217788 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

On Wed, 2 Sep 2015, Vladimir Davydov wrote:

> Slab is a kind of abnormal alloc_pages user. By calling alloc_pages_node
> with __GFP_THISNODE and w/o __GFP_WAIT before falling back to
> alloc_pages with the caller's context, it does the job normally done by
> alloc_pages itself. It's not what is done massively.
>
> Leaving slab charge path as is looks really ugly to me. Look, slab
> iterates over all nodes, inspecting if they have free pages and fails
> even if they do due to the memcg constraint...

Well yes it needs to do that due to the way NUMA support was designed in.
SLAB needs to check the per node caches if objects are present before
going to more remote nodes. Sorry about this. I realized the design issue
in 2006 and SLUB was the result in 2007 of an alternate design to let the
page allocator do its proper job.

> To sum it up. Basically, there are two ways of handling kmemcg charges:
>
>  1. Make the memcg try_charge mimic alloc_pages behavior.
>  2. Make API functions (kmalloc, etc) work in memcg as if they were
>     called from the root cgroup, while keeping interactions between the
>     low level subsys (slab) and memcg private.
>
> Way 1 might look appealing at the first glance, but at the same time it
> is much more complex, because alloc_pages has grown over the years to
> handle a lot of subtle situations that may arise on global memory
> pressure, but impossible in memcg. What does way 1 give us then? We
> can't insert try_charge directly to alloc_pages and have to spread its
> calls all over the code anyway, so why is it better? Easier to use it in
> places where users depend on buddy allocator peculiarities? There are
> not many such users.

Would it be possible to have a special alloc_pages_memcg with different
semantics?

On the other hand alloc_pages() has grown to handle all the special cases.
Why cant it also handle the special memcg case? There are numerous other
allocators that cache memory in the kernel from networking to
the bizarre compressed swap approaches. How does memcg handle that? Isnt
that situation similar to what the slab allocators do?

> exists solely for memcg-vs-list_lru and memcg-vs-slab interactions. We
> even handle kmem_cache destruction on memcg offline differently for SLAB
> and SLUB for performance reasons.

Ugly. Internal allocator design impacts container handling.

> Way 2 gives us more space to maneuver IMO. SLAB/SLUB may do weird tricks
> for optimization, but their API is well defined, so we just make kmalloc
> work as expected while providing inter-subsys calls, like
> memcg_charge_slab, for SLAB/SLUB that have their own conventions. You
> mentioned kmem users that allocate memory using alloc_pages. There is an
> API function for them too, alloc_kmem_pages. Everything behind the API
> is hidden and may be done in such a way to achieve optimal performance.

Can we also hide cgroups memory handling behind the page based schemes
without having extra handling for the slab allocators?

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#1218128 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

On Wed, Sep 02, 2015 at 01:16:47PM -0500, Christoph Lameter wrote:
> On Wed, 2 Sep 2015, Vladimir Davydov wrote:
> 
> > Slab is a kind of abnormal alloc_pages user. By calling alloc_pages_node
> > with __GFP_THISNODE and w/o __GFP_WAIT before falling back to
> > alloc_pages with the caller's context, it does the job normally done by
> > alloc_pages itself. It's not what is done massively.
> >
> > Leaving slab charge path as is looks really ugly to me. Look, slab
> > iterates over all nodes, inspecting if they have free pages and fails
> > even if they do due to the memcg constraint...
> 
> Well yes it needs to do that due to the way NUMA support was designed in.
> SLAB needs to check the per node caches if objects are present before
> going to more remote nodes. Sorry about this. I realized the design issue
> in 2006 and SLUB was the result in 2007 of an alternate design to let the
> page allocator do its proper job.

Yeah, SLUB is OK in this respect.

> 
> > To sum it up. Basically, there are two ways of handling kmemcg charges:
> >
> >  1. Make the memcg try_charge mimic alloc_pages behavior.
> >  2. Make API functions (kmalloc, etc) work in memcg as if they were
> >     called from the root cgroup, while keeping interactions between the
> >     low level subsys (slab) and memcg private.
> >
> > Way 1 might look appealing at the first glance, but at the same time it
> > is much more complex, because alloc_pages has grown over the years to
> > handle a lot of subtle situations that may arise on global memory
> > pressure, but impossible in memcg. What does way 1 give us then? We
> > can't insert try_charge directly to alloc_pages and have to spread its
> > calls all over the code anyway, so why is it better? Easier to use it in
> > places where users depend on buddy allocator peculiarities? There are
> > not many such users.
> 
> Would it be possible to have a special alloc_pages_memcg with different
> semantics?
> 
> On the other hand alloc_pages() has grown to handle all the special cases.
> Why cant it also handle the special memcg case? There are numerous other

Because we don't want to place memcg handling in alloc_pages(). AFAIU
this is because memcg by its design works at a higher layer than buddy
alloc. We can't just charge a page on alloc and uncharge it on free.
Sometimes we need to charge a page to a memcg which is different from
the current one, sometimes we need to move a page charge between cgroups
adjusting lru in the meantime (e.g. for handling readahead or swapin).
Placing memcg charging in alloc_pages() would IMO only obscure memcg
logic, because handling of the same page would be spread over subsystems
at different layers. I may be completely wrong though.

> allocators that cache memory in the kernel from networking to
> the bizarre compressed swap approaches. How does memcg handle that? Isnt

Frontswap/zswap entries are accounted to memsw counter like conventional
swap. I don't think we need to charge them to mem, because zswap size is
limited. The user allows to use some RAM as swap transparently to
running processes, so charging them to mem would be unexpected IMO.

Skbs are charged to a different counter, but not charged to kmem for
now. It is to be fixed.

> that situation similar to what the slab allocators do?

I wouldn't say so. Other users just use kmalloc or alloc_pages to grow
their buffers. kmalloc is accounted. For those who work at page
granularity and hence call alloc_pages directly, there is
alloc_kmem_pages helper.

> 
> > exists solely for memcg-vs-list_lru and memcg-vs-slab interactions. We
> > even handle kmem_cache destruction on memcg offline differently for SLAB
> > and SLUB for performance reasons.
> 
> Ugly. Internal allocator design impacts container handling.

The point is that memcg charges pages, while kmalloc works at a finer
level of granularity. As a result, we have two orthogonal strategies for
charging kmalloc:

 1. Teach memcg charge arbitrarily sized chunks and store info about
    memcg near each active object in slab.
 2. Create per memcg copy of each kmem cache (this is the scheme that is
    in use currently).

Whichever way we choose, memcg and slab have to cooperate and so slab
internal design impacts memcg handling.

> 
> > Way 2 gives us more space to maneuver IMO. SLAB/SLUB may do weird tricks
> > for optimization, but their API is well defined, so we just make kmalloc
> > work as expected while providing inter-subsys calls, like
> > memcg_charge_slab, for SLAB/SLUB that have their own conventions. You
> > mentioned kmem users that allocate memory using alloc_pages. There is an
> > API function for them too, alloc_kmem_pages. Everything behind the API
> > is hidden and may be done in such a way to achieve optimal performance.
> 
> Can we also hide cgroups memory handling behind the page based schemes
> without having extra handling for the slab allocators?
> 

I doubt so - see above.

Thanks,
Vladimir
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#1218429 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

Hello, Vladimir.

On Wed, Sep 02, 2015 at 12:30:39PM +0300, Vladimir Davydov wrote:
...
> To sum it up. Basically, there are two ways of handling kmemcg charges:
> 
>  1. Make the memcg try_charge mimic alloc_pages behavior.
>  2. Make API functions (kmalloc, etc) work in memcg as if they were
>     called from the root cgroup, while keeping interactions between the
>     low level subsys (slab) and memcg private.
> 
> Way 1 might look appealing at the first glance, but at the same time it
> is much more complex, because alloc_pages has grown over the years to
> handle a lot of subtle situations that may arise on global memory
> pressure, but impossible in memcg. What does way 1 give us then? We
> can't insert try_charge directly to alloc_pages and have to spread its
> calls all over the code anyway, so why is it better? Easier to use it in
> places where users depend on buddy allocator peculiarities? There are
> not many such users.

Maybe this is from inexperience but wouldn't 1 also be simpler than
the global case for the same reasons that doing 2 is simpler?  It's
not like the fact that memory shortage inside memcg usually doesn't
mean global shortage goes away depending on whether we take 1 or 2.

That said, it is true that slab is an integral part of kmemcg and I
can't see how it can be made oblivious of memcg operations, so yeah
one way or the other slab has to know the details and we may have to
do some unusual things at that layer.

> I understand that the idea of way 1 is to provide a well-defined memcg
> API independent of the rest of the code, but that's just impossible. You
> need special casing anyway. E.g. you need those get/put_kmem_cache
> helpers, which exist solely for SLAB/SLUB. You need all this special
> stuff for growing per-memcg array in list_lru and kmem_cache, which
> exists solely for memcg-vs-list_lru and memcg-vs-slab interactions. We
> even handle kmem_cache destruction on memcg offline differently for SLAB
> and SLUB for performance reasons.

It isn't a black or white thing.  Sure, slab should be involved in
kmemcg but at the same time if we can keep the amount of exposure in
check, that's the better way to go.

> Way 2 gives us more space to maneuver IMO. SLAB/SLUB may do weird tricks
> for optimization, but their API is well defined, so we just make kmalloc
> work as expected while providing inter-subsys calls, like
> memcg_charge_slab, for SLAB/SLUB that have their own conventions. You
> mentioned kmem users that allocate memory using alloc_pages. There is an
> API function for them too, alloc_kmem_pages. Everything behind the API
> is hidden and may be done in such a way to achieve optimal performance.

Ditto.  Nobody is arguing that we can get it out completely but at the
same time handling of GFP_NOWAIT seems like a pretty fundamental
proprety that we'd wanna maintain at memcg boundary.

You said elsewhere that GFP_NOWAIT happening back-to-back is unlikely.
I'm not sure how much we can commit to that statement.  GFP_KERNEL
allocating huge amount of memory in a single go is a kernel bug.
GFP_NOWAIT optimization in a hot path which is accessible to userland
isn't and we'll be growing more and more of them.  We need to be
protected against back-to-back GFP_NOWAIT allocations.

Thanks.

-- 
tejun
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#1218832 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

On Thu, Sep 03, 2015 at 12:32:43PM -0400, Tejun Heo wrote:
> On Wed, Sep 02, 2015 at 12:30:39PM +0300, Vladimir Davydov wrote:
> ...
> > To sum it up. Basically, there are two ways of handling kmemcg charges:
> > 
> >  1. Make the memcg try_charge mimic alloc_pages behavior.
> >  2. Make API functions (kmalloc, etc) work in memcg as if they were
> >     called from the root cgroup, while keeping interactions between the
> >     low level subsys (slab) and memcg private.
> > 
> > Way 1 might look appealing at the first glance, but at the same time it
> > is much more complex, because alloc_pages has grown over the years to
> > handle a lot of subtle situations that may arise on global memory
> > pressure, but impossible in memcg. What does way 1 give us then? We
> > can't insert try_charge directly to alloc_pages and have to spread its
> > calls all over the code anyway, so why is it better? Easier to use it in
> > places where users depend on buddy allocator peculiarities? There are
> > not many such users.
> 
> Maybe this is from inexperience but wouldn't 1 also be simpler than
> the global case for the same reasons that doing 2 is simpler?  It's
> not like the fact that memory shortage inside memcg usually doesn't
> mean global shortage goes away depending on whether we take 1 or 2.
> 
> That said, it is true that slab is an integral part of kmemcg and I
> can't see how it can be made oblivious of memcg operations, so yeah
> one way or the other slab has to know the details and we may have to
> do some unusual things at that layer.
> 
> > I understand that the idea of way 1 is to provide a well-defined memcg
> > API independent of the rest of the code, but that's just impossible. You
> > need special casing anyway. E.g. you need those get/put_kmem_cache
> > helpers, which exist solely for SLAB/SLUB. You need all this special
> > stuff for growing per-memcg array in list_lru and kmem_cache, which
> > exists solely for memcg-vs-list_lru and memcg-vs-slab interactions. We
> > even handle kmem_cache destruction on memcg offline differently for SLAB
> > and SLUB for performance reasons.
> 
> It isn't a black or white thing.  Sure, slab should be involved in
> kmemcg but at the same time if we can keep the amount of exposure in
> check, that's the better way to go.
> 
> > Way 2 gives us more space to maneuver IMO. SLAB/SLUB may do weird tricks
> > for optimization, but their API is well defined, so we just make kmalloc
> > work as expected while providing inter-subsys calls, like
> > memcg_charge_slab, for SLAB/SLUB that have their own conventions. You
> > mentioned kmem users that allocate memory using alloc_pages. There is an
> > API function for them too, alloc_kmem_pages. Everything behind the API
> > is hidden and may be done in such a way to achieve optimal performance.
> 
> Ditto.  Nobody is arguing that we can get it out completely but at the
> same time handling of GFP_NOWAIT seems like a pretty fundamental
> proprety that we'd wanna maintain at memcg boundary.

Agree, but SLAB/SLUB aren't just calling GFP_NOWAIT. They're doing
pretty low level tricks, which aren't common for the rest of the system.

Inspecting all nodes with __GFP_THISNODE and w/o __GFP_WAIT before
calling reclaimer is what can and should be done by buddy allocator.
I've never seen anyone doing things like this apart from SLAB (note SLUB
doesn't do this). SLAB does this for historical reasons. We could fix
it, but that would require rewriting SLAB code to a great extent, which
isn't preferable, because we can easily break something.

Trying a high-order page before falling back on lower order is not
something really common. It implicitly relies on the fact that
reclaiming memory for a new continuous high-order page is much more
expensive than getting the same amount of order-1 pages. This is true
for buddy alloc, but not for memcg. That's why playing such a trick with
try_charge is wrong IMO. If such a trick becomes common, I think we will
have to introduce a helper for it, because otherwise a change in buddy
alloc internal logic (e.g. a defrag optimization making high order pages
cheaper) may affect its users.

That said, I totally agree that memcg should handle GFP_NOWAIT, but I'm
opposed to the idea that it should handle the tricks that rely on
internal buddy alloc logic similar to those used by SLAB and SLUB. We'd
better strive to hide these tricks in buddy alloc helpers and never use
them directly.

That's why I think we need these patches and they aren't workarounds
that can be reverted once try_charge has been taught to handle
GFP_NOWAIT properly.

> 
> You said elsewhere that GFP_NOWAIT happening back-to-back is unlikely.
> I'm not sure how much we can commit to that statement.  GFP_KERNEL
> allocating huge amount of memory in a single go is a kernel bug.
> GFP_NOWAIT optimization in a hot path which is accessible to userland
> isn't and we'll be growing more and more of them.  We need to be
> protected against back-to-back GFP_NOWAIT allocations.

AFAIU if someone tries to allocate with GFP_NOWAIT (i.e. w/o
__GFP_NOFAIL or __GFP_HIGH), he/she must be prepared to allocation
failures, so there should be a safe fall back path, which fixes things
in normal context. It doesn't mean we shouldn't do anything to satisfy
such optimistic requests from memcg, but we may occasionally fail them.

OTOH if someone allocates with GFP_KERNEL, he/she should be prepared to
get NULL, but in this case the whole operation will usually be aborted.
Therefore with the possibility of all GFP_KERNEL being transformed to
GFP_NOWAIT inside slab, memcg has to be extra cautious, because failing
a usual GFP_NOWAIT in such a case may result not in falling back on slow
path, but in user-visible effects like failing to open a file with
ENOMEM. This is really difficult to achieve and I doubt it's worth
complicating memcg code, because we can just fix SLAB/SLUB.

Regarding __GFP_NOFAIL and __GFP_HIGH, IMO we can let them go uncharged
or charge them forcefully even if they breach the limit, because there
shouldn't be many of them (if there were really a lot of them, they
could deplete memory reserves and hang the system).

If all these assumptions are true, we don't need to do anything (apart
from forcefully charging high prio allocations may be) for kmemcg to
work satisfactory. For optimizing optimistic GFP_NOWAIT callers one can
use memory.high instead or along with memory.max. Reclaiming memory.high
in kernel while holding various locks can result in prio inversions
though, but that's a different story, which could be fixed by task_work
reclaim.

I admit I may be mistaken, but if I'm right, we may end up with really
complex memcg reclaim logic trying to closely mimic behavior of buddy
alloc with all its historic peculiarities. That's why I don't want to
rush ahead "fixing" memcg reclaim before an agreement among all
interested people is reached...

Thanks,
Vladimir
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#1219133 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

Hello, Vladimir.

On Fri, Sep 04, 2015 at 02:15:50PM +0300, Vladimir Davydov wrote:
> Trying a high-order page before falling back on lower order is not
> something really common. It implicitly relies on the fact that
> reclaiming memory for a new continuous high-order page is much more
> expensive than getting the same amount of order-1 pages. This is true
> for buddy alloc, but not for memcg. That's why playing such a trick with
> try_charge is wrong IMO. If such a trick becomes common, I think we will
> have to introduce a helper for it, because otherwise a change in buddy
> alloc internal logic (e.g. a defrag optimization making high order pages
> cheaper) may affect its users.

I'm having trouble following why this matters.  The layering here is
pretty clear regardless of how slab is trespassing into page
allocator's role.  memcg of course doesn't care whether an allocation
is high-order or order-1.  All it does is imposing extra restrictions
when allocating memory and all that's necessary is reasonably
satisfying the expectations expressed by the specified gfp mask.

> That said, I totally agree that memcg should handle GFP_NOWAIT, but I'm
> opposed to the idea that it should handle the tricks that rely on
> internal buddy alloc logic similar to those used by SLAB and SLUB. We'd
> better strive to hide these tricks in buddy alloc helpers and never use
> them directly.

All these don't really matter once memcg handles GFP_NOWAIT in a
reasonable manner, right?  memcg doesn't need all the fancy tricks of
the page allocator.  All it needs is honoring the intentions expressed
by the gfp mask in a reasonable way w/o systematic failures.
 
> That's why I think we need these patches and they aren't workarounds
> that can be reverted once try_charge has been taught to handle
> GFP_NOWAIT properly.

So, if this is separate slab improvements, I have no objections but
independent of that, we need to be able to handle back-to-back
GFP_NOWAIT cases and w/ the high limit punting to the return path
should work well enough.

> > You said elsewhere that GFP_NOWAIT happening back-to-back is unlikely.
> > I'm not sure how much we can commit to that statement.  GFP_KERNEL
> > allocating huge amount of memory in a single go is a kernel bug.
> > GFP_NOWAIT optimization in a hot path which is accessible to userland
> > isn't and we'll be growing more and more of them.  We need to be
> > protected against back-to-back GFP_NOWAIT allocations.
> 
> AFAIU if someone tries to allocate with GFP_NOWAIT (i.e. w/o
> __GFP_NOFAIL or __GFP_HIGH), he/she must be prepared to allocation
> failures, so there should be a safe fall back path, which fixes things
> in normal context. It doesn't mean we shouldn't do anything to satisfy
> such optimistic requests from memcg, but we may occasionally fail them.

Yes, it can fail under stress or if unluckly; however, it shouldn't
fail consistently under nominal conditions or be able to run over high
limit unchecked.

> OTOH if someone allocates with GFP_KERNEL, he/she should be prepared to
> get NULL, but in this case the whole operation will usually be aborted.
> Therefore with the possibility of all GFP_KERNEL being transformed to
> GFP_NOWAIT inside slab, memcg has to be extra cautious, because failing
> a usual GFP_NOWAIT in such a case may result not in falling back on slow
> path, but in user-visible effects like failing to open a file with
> ENOMEM. This is really difficult to achieve and I doubt it's worth
> complicating memcg code, because we can just fix SLAB/SLUB.

I'm not following you at all here.  slab too of course should fall
back to more robust gfp mask if NOWAIT fails and as long as those
failures are exceptions, it's fine.

> Regarding __GFP_NOFAIL and __GFP_HIGH, IMO we can let them go uncharged
> or charge them forcefully even if they breach the limit, because there
> shouldn't be many of them (if there were really a lot of them, they
> could deplete memory reserves and hang the system).
> 
> If all these assumptions are true, we don't need to do anything (apart
> from forcefully charging high prio allocations may be) for kmemcg to
> work satisfactory. For optimizing optimistic GFP_NOWAIT callers one can
> use memory.high instead or along with memory.max. Reclaiming memory.high
> in kernel while holding various locks can result in prio inversions
> though, but that's a different story, which could be fixed by task_work
> reclaim.

GFP_NOWAIT has a systematic problem which needs to be fixed.

> I admit I may be mistaken, but if I'm right, we may end up with really
> complex memcg reclaim logic trying to closely mimic behavior of buddy
> alloc with all its historic peculiarities. That's why I don't want to
> rush ahead "fixing" memcg reclaim before an agreement among all
> interested people is reached...

I think that's a bit out of proportion.  I'm not suggesting bringing
in all complexities of global reclaim.  There's no reason to and what
memcg deals with is inherently way simpler than actual memory
allocation.  The original patch was about fixing systematic failure
around GFP_NOWAIT close to the high limit.  We might want to do
background reclaim close to max but as long as high limit functions
correctly, that's much less of a problem at least on the v2 interface.

Thanks.

-- 
tejun
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#1219237 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

Hi Tejun, Michal

On Fri, Sep 04, 2015 at 11:44:48AM -0400, Tejun Heo wrote:
...
> > I admit I may be mistaken, but if I'm right, we may end up with really
> > complex memcg reclaim logic trying to closely mimic behavior of buddy
> > alloc with all its historic peculiarities. That's why I don't want to
> > rush ahead "fixing" memcg reclaim before an agreement among all
> > interested people is reached...
> 
> I think that's a bit out of proportion.  I'm not suggesting bringing
> in all complexities of global reclaim.  There's no reason to and what
> memcg deals with is inherently way simpler than actual memory
> allocation.  The original patch was about fixing systematic failure
> around GFP_NOWAIT close to the high limit.  We might want to do
> background reclaim close to max but as long as high limit functions
> correctly, that's much less of a problem at least on the v2 interface.

Looking through this thread once again and weighting my arguments vs
yours, I start to understand that I'm totally wrong and these patches
are not proper fixes for the problem.

Having these patches in the kernel only helps when we are hitting the
hard limit, which shouldn't occur often if memory.high works properly.
Even if memory.high is not used, the only negative effect we would get
w/o them is allocating a slab from a wrong node or getting a low order
page where we could get a high order one. Both should be rare and both
aren't critical. I think I got carried away with all those obscure
"reclaimer peculiarities" at some point.

Now I think task_work reclaim initially proposed by Tejun would be a
much better fix.

I'm terribly sorry for being so annoying and stubborn and want to thank
you for all your feedback!

Thanks,
Vladimir
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#1219269 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

Hello, Vladimir.

On Fri, Sep 04, 2015 at 09:21:11PM +0300, Vladimir Davydov wrote:
> Now I think task_work reclaim initially proposed by Tejun would be a
> much better fix.

Cool, I'll update the patch.

> I'm terribly sorry for being so annoying and stubborn and want to thank
> you for all your feedback!

Heh, I'm not all that confident about my position.  A lot of it could
be from lack of experience and failing to see the gradients.  Please
keep me in check if I get lost.

Thanks a lot!

-- 
tejun
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#1219094 — Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled

On Wed 02-09-15 12:30:39, Vladimir Davydov wrote:
> [
>   I'll try to summarize my point in one hunk instead of spreading it all
>   over the e-mail, because IMO it's becoming a kind of difficult to
>   follow. If you think that there's a question I dodge, please let me
>   now and I'll try to address it separately.
> 
>   Also, adding Johannes to Cc (I noticed that I accidentally left him
>   out), because this discussion seems to be fundamental and may affect
>   our further steps dramatically.
> ]
> 
> On Tue, Sep 01, 2015 at 08:38:50PM +0200, Michal Hocko wrote:
[...]
> > I guess we are still not at the same page here. If the slab has a subtle
> > behavior (and from what you are saying it seems it has the same behavior
> > at the global scope) then we should strive to fix it rather than making
> > it more obscure just to not expose GFP_NOWAIT to memcg which is not
> > handled properly currently wrt. high limit (more on that below) which
> > was the primary motivation for the patch AFAIU.
> 
> Slab is a kind of abnormal alloc_pages user. By calling alloc_pages_node
> with __GFP_THISNODE and w/o __GFP_WAIT before falling back to
> alloc_pages with the caller's context, it does the job normally done by
> alloc_pages itself. It's not what is done massively.
> 
> Leaving slab charge path as is looks really ugly to me. Look, slab
> iterates over all nodes, inspecting if they have free pages and fails
> even if they do due to the memcg constraint...

Yes, I understand what you are saying. The way how SLAB does its thing
is really subtle. The special combination of flags even prevents the
background reclaim which is weird. There was probably a good reason for
that but the point I've tried to make is that if the heuristic relies on
non-reclaiming behavior for the global case then the memcg should copy
that as much as possible. The allocator has to be prepared for the
non-sleeping allocation failure and the fact that memcg causes it sooner
is just natural because that is what the memcg is used for.

I see how you try to optimize around this subtle behavior but that only
makes it even more subtle long term.

> My point is that what slab does is a pretty low level thing, normal
> users call alloc_pages or kmalloc with flags corresponding to their
> context. Of course, there may be special users trying optimistically
> GFP_NOWAIT, but they aren't massive, and that simplifies things for
> memcg a lot.

memcg code _absolutely_ has to deal with NOWAIT requests somehow. I can
see more and more of them coming long term. Because it makes a lot of
sense to do an opportunistic allocation with a fallback. And that was
the whole point. You have started by tweaking SL.B whereas memcg is
where we should start see the resulting behavior and then think about
SL.B specific fix.

> I mean if we can rely on the fact that the number of
> GFP_NOWAIT allocations that can occur in a row is limited we can use
> direct reclaim (like memory.high) and/or task_work reclaim to fix
> GFP_NOWAIT failures. Otherwise, we have to mimic the global alloc with
> most its heuristics. I don't think that copying those heuristics is the
> right thing to do, because in memcg case the same problems may be
> resolved much easier, because we don't actually experience real memory
> shortage when hitting the limit.

I am not really sure I understand what you mean here. What kind of
heuristics you have in mind? All that memcg code cares about is the keep
high limit contained and converge as much as possible.
 
> Moreover, we already treat some flags not in the same way as in case of
> slab for simplicity. E.g. we let __GFP_NOFAIL allocations go uncharged
> instead of retrying infinitely.

Yes we rely on the global MM to handle those. Which is a reasonable
compromise IMO. Such a strong liability cannot realistically be handled
inside memcg without causing more problems.

> We ignore __GFP_THISNODE thing and we just cannot take it into account.

yes because it is allocation and not reclaim related mode. There is a
reason it is not part of GFP_RECLAIM_MASK.

> We ignore allocation order, because that makes no sense for memcg.

We are not ignoring it completely because we base our reclaim target on
it.

> To sum it up. Basically, there are two ways of handling kmemcg charges:
> 
>  1. Make the memcg try_charge mimic alloc_pages behavior.
>  2. Make API functions (kmalloc, etc) work in memcg as if they were
>     called from the root cgroup, while keeping interactions between the
>     low level subsys (slab) and memcg private.
> 
> Way 1 might look appealing at the first glance, but at the same time it
> is much more complex, because alloc_pages has grown over the years to
> handle a lot of subtle situations that may arise on global memory
> pressure, but impossible in memcg. What does way 1 give us then? We
> can't insert try_charge directly to alloc_pages and have to spread its
> calls all over the code anyway, so why is it better? Easier to use it in
> places where users depend on buddy allocator peculiarities? There are
> not many such users.

Because the more consistent allocation and charging paths are in the
reclaim behavior the easier will be the system to understand and maintain.

> I understand that the idea of way 1 is to provide a well-defined memcg
> API independent of the rest of the code, but that's just impossible. You
> need special casing anyway. E.g. you need those get/put_kmem_cache
> helpers, which exist solely for SLAB/SLUB. You need all this special
> stuff for growing per-memcg array in list_lru and kmem_cache, which
> exists solely for memcg-vs-list_lru and memcg-vs-slab interactions. We
> even handle kmem_cache destruction on memcg offline differently for SLAB
> and SLUB for performance reasons.
> 
> Way 2 gives us more space to maneuver IMO. SLAB/SLUB may do weird tricks
> for optimization, but their API is well defined, so we just make kmalloc
> work as expected while providing inter-subsys calls, like
> memcg_charge_slab, for SLAB/SLUB that have their own conventions.

I do agree that we might end up needing SL.B specific hacks but, again,
let's get there only when we see that the memcg code cannot cope with
it by default. E.g. our currently non-existing NOWAIT logic would fail
too often because of the high limit which would lead to non optimal NUMA
behavior of SLAB.

> You mentioned kmem users that allocate memory using alloc_pages. There
> is an API function for them too, alloc_kmem_pages. Everything behind
> the API is hidden and may be done in such a way to achieve optimal
> performance.
-- 
Michal Hocko
SUSE Labs
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