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Re: [PATCH 2/4] oom: Do not invoke oom notifiers on sysrq+f

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  Re: [PATCH 2/4] oom: Do not invoke oom notifiers on sysrq+f David Rientjes <rientjes@google.com> - 2015-07-09 01:40 +0200
    Re: [PATCH 2/4] oom: Do not invoke oom notifiers on sysrq+f Michal Hocko <mhocko@kernel.org> - 2015-07-09 11:00 +0200
      Re: [PATCH 2/4] oom: Do not invoke oom notifiers on sysrq+f Michal Hocko <mhocko@kernel.org> - 2015-07-15 11:50 +0200

#1180303 — Re: [PATCH 2/4] oom: Do not invoke oom notifiers on sysrq+f

On Wed, 8 Jul 2015, Michal Hocko wrote:

> From: Michal Hocko <mhocko@suse.cz>
> 
> A github user rfjakob has reported the following issue via IRC.
> <rfjakob> Manually triggering the OOM killer does not work anymore in 4.0.5
> <rfjakob> This is what it looks like: https://gist.github.com/rfjakob/346b7dc611fc3cdf4011
> <rfjakob> Basically, what happens is that the GPU driver frees some memory, that satisfies the OOM killer
> <rfjakob> But the memory is allocated immediately again, and in the, no processes are killed no matter how often you trigger the oom killer
> <rfjakob> "in the end"
> 
> Quoting from the github:
> "
> [19291.202062] sysrq: SysRq : Manual OOM execution
> [19291.208335] Purging GPU memory, 74399744 bytes freed, 8728576 bytes still pinned.
> [19291.390767] sysrq: SysRq : Manual OOM execution
> [19291.396792] Purging GPU memory, 74452992 bytes freed, 8728576 bytes still pinned.
> [19291.560349] sysrq: SysRq : Manual OOM execution
> [19291.566018] Purging GPU memory, 75489280 bytes freed, 8728576 bytes still pinned.
> [19291.729944] sysrq: SysRq : Manual OOM execution
> [19291.735686] Purging GPU memory, 74399744 bytes freed, 8728576 bytes still pinned.
> [19291.918637] sysrq: SysRq : Manual OOM execution
> [19291.924299] Purging GPU memory, 74403840 bytes freed, 8728576 bytes still pinned.
> "
> 
> The issue is that sysrq+f (force_kill) gets confused by the regular OOM
> heuristic which tries to prevent from OOM killer if some of the oom
> notifier can relase a memory. The heuristic doesn't make much sense for
> the sysrq+f path because this one is used by the administrator to kill
> a memory hog.
> 
> Reported-by: Jakob Unterwurzacher <jakobunt@gmail.com>
> Signed-off-by: Michal Hocko <mhocko@suse.cz>

Nack, the oom notify list has no place in the oom killer, it should be 
called in the page allocator before calling out_of_memory().  
out_of_memory() should serve a single, well defined purpose: kill a 
process.  If this were done, you wouldn't need random hacks like this in 
place.  This also shouldn't be included in a patchset that redefines the 
semantics of a forced oom kill, which is quite separate.
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#1180563

On Wed 08-07-15 16:37:49, David Rientjes wrote:
> On Wed, 8 Jul 2015, Michal Hocko wrote:
> 
> > From: Michal Hocko <mhocko@suse.cz>
> > 
> > A github user rfjakob has reported the following issue via IRC.
> > <rfjakob> Manually triggering the OOM killer does not work anymore in 4.0.5
> > <rfjakob> This is what it looks like: https://gist.github.com/rfjakob/346b7dc611fc3cdf4011
> > <rfjakob> Basically, what happens is that the GPU driver frees some memory, that satisfies the OOM killer
> > <rfjakob> But the memory is allocated immediately again, and in the, no processes are killed no matter how often you trigger the oom killer
> > <rfjakob> "in the end"
> > 
> > Quoting from the github:
> > "
> > [19291.202062] sysrq: SysRq : Manual OOM execution
> > [19291.208335] Purging GPU memory, 74399744 bytes freed, 8728576 bytes still pinned.
> > [19291.390767] sysrq: SysRq : Manual OOM execution
> > [19291.396792] Purging GPU memory, 74452992 bytes freed, 8728576 bytes still pinned.
> > [19291.560349] sysrq: SysRq : Manual OOM execution
> > [19291.566018] Purging GPU memory, 75489280 bytes freed, 8728576 bytes still pinned.
> > [19291.729944] sysrq: SysRq : Manual OOM execution
> > [19291.735686] Purging GPU memory, 74399744 bytes freed, 8728576 bytes still pinned.
> > [19291.918637] sysrq: SysRq : Manual OOM execution
> > [19291.924299] Purging GPU memory, 74403840 bytes freed, 8728576 bytes still pinned.
> > "
> > 
> > The issue is that sysrq+f (force_kill) gets confused by the regular OOM
> > heuristic which tries to prevent from OOM killer if some of the oom
> > notifier can relase a memory. The heuristic doesn't make much sense for
> > the sysrq+f path because this one is used by the administrator to kill
> > a memory hog.
> > 
> > Reported-by: Jakob Unterwurzacher <jakobunt@gmail.com>
> > Signed-off-by: Michal Hocko <mhocko@suse.cz>
> 
> Nack, the oom notify list has no place in the oom killer, it should be 
> called in the page allocator before calling out_of_memory().  

I cannot say I would like oom notifiers interface. Quite contrary, it is
just a crude hack. It is living outside of the shrinker interface which is
what the reclaim is using and it acts like the last attempt before OOM
(e.g. i915_gem_shrinker_init registers both "shrinkers"). So I am not
sure it belongs outside of the oom killer proper.

Besides that out_of_memory already contains shortcuts to prevent killing
a task. Why is this any different? I mean why shouldn't callers of
out_of_memory check whether the task is killed or existing before
calling out_of_memory?

-- 
Michal Hocko
SUSE Labs
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#1184553

On Tue 14-07-15 14:58:55, David Rientjes wrote:
> On Fri, 10 Jul 2015, Michal Hocko wrote:
> 
> > Shrinkers are there to reclaim and prevent from OOM. This API is a gray
> > zone. It looks generic method for the notification yet it allows to
> > prevent from oom killer. I can imagine somebody might abuse this
> > interface to implement OOM killer policies.
> > 
> > Anyway, I think it would be preferable to kill it altogether rather than
> > play with its placing. It will always be a questionable API.
> > 
> 
> Agreed.

In such a case it would be still good to fix the bug fixed by this
patch.

-- 
Michal Hocko
SUSE Labs
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