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

From	Mike Galbraith <umgwanakikbuti@gmail.com>
Newsgroups	linux.kernel
Subject	Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable again and shut down on idle)
Date	2016-01-26 03:20 +0100
Message-ID	<qV1oB-4JB-1@gated-at.bofh.it> (permalink)
References	(7 earlier) <qTMBk-6ri-29@gated-at.bofh.it> <qTN4n-6Dx-31@gated-at.bofh.it> <qU9ey-5ik-15@gated-at.bofh.it> <qUTr3-7vm-11@gated-at.bofh.it> <qUTKq-7UQ-17@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

On Mon, 2016-01-25 at 12:02 -0600, 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?

Because we schedule to idle cores aggressively, thus we may pop in and
out of idle at high frequency.

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

Again, the cross core activity is not due to quiet_vmstat(), it is due
to pipe-test threads running on two cores, and meeting quiet_vmstat()
at high frequency.

> > 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.

Perf and RT say we don't want quiet_vmstat() in the idle loop either.

	-Mike

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

csiph-web