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Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable again and shut down on idle)

From Michal Hocko <mhocko@kernel.org>
Newsgroups linux.kernel
Subject Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable again and shut down on idle)
Date 2016-01-25 21:20 +0100
Message-ID <qUVMf-PD-23@gated-at.bofh.it> (permalink)
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On Mon 25-01-16 12:02:06, Christoph Lameter wrote:
> On Mon, 25 Jan 2016, Michal Hocko wrote:
> 
> > On Sat 23-01-16 17:21:55, Mike Galbraith wrote:
> > > Hi Christoph,
> > >
> > > While you're fixing that commit up, can you perhaps find a better home
> > > for quiet_vmstat()?  It not only munches cycles when switching cross
> > > -core mightily, for -rt it injects a sleeping lock into the idle task.
> > >
> > >     12.89%  [kernel]       [k] refresh_cpu_vm_stats.isra.12
> > >      4.75%  [kernel]       [k] __schedule
> > >      4.70%  [kernel]       [k] mutex_unlock
> > >      3.14%  [kernel]       [k] __switch_to
> >
> > Hmm, I wouldn't have expected that refresh_cpu_vm_stats could have
> > such a large footprint. I guess this would be just an expensive noop
> > because we have to check all the zones*counters and do an expensive
> > this_cpu_xchg. Is the whole deferred thing worth this overhead?
> 
> Why would the deferring cause this overhead?

I guess the profile speaks for itself, doesn't it?

> Also there is no cross core activity from quiet_vmstat(). It simply
> disables the local vmstat updates.

It doesn't go cross core but it still does nr_zones * counters atomic
ops.

> > Unless there is a clear and huge win from doing the vmstat update
> > deferrable then I think a revert is more appropriate IMHO.
> 
> It reduces the OS events that the application experiences by folding it
> into the tick events. If its not deferrable then a timer event will be
> generated in addition to the tick. We do not want that.

Yes this is what I have read in the changelog. But "how much" part is
really missing. Is this even quantifiable?

> Workqueues are used in many places. If RT can sleep within workqueue
> management functions then spinlocks cannot be taken anymore and there may
> be issues with preemption.

RT can sleep in _any_ spinlock except for raw spin locks. Even though
the !RT kernel is not sleeping doesn't really matter much because
cancel_delayed_work is quite a heavy function which shouldn't be called
from the idle context AFAIU. Sure most of the time it will boil down to
del_timer but it can hit the slowpath as well if the timer got migrated
to a different CPU and we have to race with the WQ pool management IIUC.

Maybe this overhead can be reduced by outsourcing the functionality to
vmstat_shepherd which can check idle CPUs, cancel the timer for them
update the differentials and put them to cpu_stat_off? 

> The regression that I know of (independent of "RT") is due as far as I
> know due to the switch of the parameters of some vmstat functions to 64
> bit instead of 32 bit.

I am not sure I am following.

-- 
Michal Hocko
SUSE Labs

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Thread

Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-20 15:40 +0100
  Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Sasha Levin <sasha.levin@oracle.com> - 2016-01-20 16:00 +0100
    Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-20 16:20 +0100
      Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Christoph Lameter <cl@linux.com> - 2016-01-20 16:30 +0100
        Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Sasha Levin <sasha.levin@oracle.com> - 2016-01-20 17:00 +0100
          Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Christoph Lameter <cl@linux.com> - 2016-01-20 17:00 +0100
            Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-20 22:30 +0100
              Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Christoph Lameter <cl@linux.com> - 2016-01-20 23:00 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-21 09:30 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Christoph Lameter <cl@linux.com> - 2016-01-21 16:50 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-21 18:00 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Christoph Lameter <cl@linux.com> - 2016-01-21 18:40 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Shiraz Hashim <shiraz.linux.kernel@gmail.com> - 2016-01-22 12:10 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-22 15:10 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Christoph Lameter <cl@linux.com> - 2016-01-22 17:10 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-22 17:20 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Christoph Lameter <cl@linux.com> - 2016-01-22 17:50 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-22 18:20 +0100
                fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-23 17:30 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-24 01:40 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-24 03:50 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-24 04:50 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-24 06:40 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Michal Hocko <mhocko@kernel.org> - 2016-01-25 18:50 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-25 19:10 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Michal Hocko <mhocko@kernel.org> - 2016-01-25 21:20 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-26 17:30 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-26 19:40 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-26 19:50 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-26 20:30 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-27 04:20 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-27 05:20 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-27 17:30 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-26 19:40 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-26 03:20 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-26 03:30 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-26 17:30 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-26 18:40 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-26 19:20 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-26 17:30 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-26 18:10 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-26 19:30 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-26 20:10 +0100
                Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable  again and shut down on idle) Christoph Lameter <cl@linux.com> - 2016-01-26 20:30 +0100
                [PATCH] mm, vmstat: make quiet_vmstat lighter (was: Re: fast path  cycle muncher (vmstat: make vmstat_updater deferrable) again and shut down  on idle) Michal Hocko <mhocko@kernel.org> - 2016-01-27 17:50 +0100
                Re: [PATCH] mm, vmstat: make quiet_vmstat lighter (was: Re: fast  path cycle muncher (vmstat: make vmstat_updater deferrable) again and shut  down on idle) Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-01-27 18:10 +0100
                Re: [PATCH] mm, vmstat: make quiet_vmstat lighter (was: Re: fast  path cycle muncher (vmstat: make vmstat_updater deferrable) again and shut  down on idle) Christoph Lameter <cl@linux.com> - 2016-01-27 19:30 +0100
                Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-24 18:00 +0100
  Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Christoph Lameter <cl@linux.com> - 2016-01-20 16:20 +0100
    Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Michal Hocko <mhocko@kernel.org> - 2016-01-20 16:30 +0100

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