Re: sched: tweak select_idle_sibling to look for idle threads

From	Mike Galbraith <mgalbraith@suse.de>
Newsgroups	linux.kernel
Subject	Re: sched: tweak select_idle_sibling to look for idle threads
Date	2016-05-09 09:50 +0200
Message-ID	<rwO71-yr-43@gated-at.bofh.it> (permalink)
References	(6 earlier) <rw6pj-7ak-3@gated-at.bofh.it> <rwrWO-37i-3@gated-at.bofh.it> <rwJgZ-2Lv-5@gated-at.bofh.it> <rwKmK-3VJ-7@gated-at.bofh.it> <rwKG6-4Ch-1@gated-at.bofh.it>
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On Mon, 2016-05-09 at 04:22 +0800, Yuyang Du wrote:
> On Mon, May 09, 2016 at 05:45:40AM +0200, Mike Galbraith wrote:
> > On Mon, 2016-05-09 at 02:57 +0800, Yuyang Du wrote:
> > > On Sun, May 08, 2016 at 10:08:55AM +0200, Mike Galbraith wrote:
> > > > > Maybe give the criteria a bit margin, not just wakees tend to equal llc_size,
> > > > > but the numbers are so wild to easily break the fragile condition, like:
> > > > 
> > > > Seems lockless traversal and averages just lets multiple CPUs select
> > > > the same spot.  An atomic reservation (feature) when looking for an
> > > > idle spot (also for fork) might fix it up.  Run the thing as RT,
> > > > push/pull ensures that it reaches box saturation regardless of the
> > > > number of messaging threads, whereas with fair class, any number > 1
> > > > will certainly stack tasks before the box is saturated.
> > > 
> > > Yes, good idea, bringing order to the race to grab idle CPU is absolutely
> > > helpful.
> > 
> > Well, good ideas work, as yet this one helps jack diddly spit.
> 
> Then a valid question is whether it is this selection screwed up in case
> like this, as it should necessarily always be asked.

That's a given, it's just a question of how to do a bit better cheaply.
 
> > > Regarding wake_wide(), it seems the M:N is 1:24, not 6:6*24, if so,
> > > the slave will be 0 forever (as last_wakee is never flipped).
> > 
> > Yeah, it's irrelevant here, this load is all about instantaneous state.
> >  I could use a bit more of that, reserving on the wakeup side won't
> > help this benchmark until everything else cares.  One stack, and it's
> > game over.  It could help generic utilization and latency some.. but it
> > seems kinda unlikely it'll be worth the cycle expenditure.
> 
> Yes and no, it depends on how efficient work-stealing is, compared to
> selection, but remember, at the end of the day, the wakee CPU measures the
> latency, that CPU does not care it is selected or it steals.

In a perfect world, running only Chris' benchmark on an otherwise idle
box, there would never _be_ any work to steal.  In the real world, we
smooth utilization, optimistically peek at this/that, and intentionally
throttle idle balancing (etc etc), which adds up to an imperfect world
for this (based on real world load) benchmark.

> En... should we try remove recording last_wakee?

The more the merrier, go for it! :)

	-Mike

Thread

Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-04-30 14:50 +0200
  Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-01 09:20 +0200
    Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-01 11:00 +0200
      Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-01 11:30 +0200
        Re: sched: tweak select_idle_sibling to look for idle threads Yuyang Du <yuyang.du@intel.com> - 2016-05-07 11:10 +0200
          Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-08 10:10 +0200
            Re: sched: tweak select_idle_sibling to look for idle threads Yuyang Du <yuyang.du@intel.com> - 2016-05-09 04:40 +0200
              Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-09 05:50 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Yuyang Du <yuyang.du@intel.com> - 2016-05-09 06:10 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-09 09:50 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Yuyang Du <yuyang.du@intel.com> - 2016-05-09 11:00 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-09 11:40 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Yuyang Du <yuyang.du@intel.com> - 2016-05-10 09:10 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-10 09:50 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-05-10 17:30 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Yuyang Du <yuyang.du@intel.com> - 2016-05-11 05:00 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-05-11 06:20 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Yuyang Du <yuyang.du@intel.com> - 2016-05-11 11:10 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-05-11 12:00 +0200
              Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-09 06:00 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Yuyang Du <yuyang.du@intel.com> - 2016-05-09 06:20 +0200
    Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-02 10:50 +0200
      Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-02 17:00 +0200
        Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-02 17:00 +0200
          Re: sched: tweak select_idle_sibling to look for idle threads Chris Mason <clm@fb.com> - 2016-05-02 17:50 +0200
            Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-03 16:40 +0200
              Re: sched: tweak select_idle_sibling to look for idle threads Chris Mason <clm@fb.com> - 2016-05-03 17:20 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-04 12:40 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-04 17:40 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Matt Fleming <matt@codeblueprint.co.uk> - 2016-05-06 00:10 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-06 21:00 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-09 10:40 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-09 11:00 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-04 17:50 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Chris Mason <clm@fb.com> - 2016-05-04 19:50 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-05 11:40 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Chris Mason <clm@fb.com> - 2016-05-05 16:00 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-06 09:20 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Chris Mason <clm@fb.com> - 2016-05-06 19:30 +0200
                Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-06 09:30 +0200
          Re: sched: tweak select_idle_sibling to look for idle threads Mike Galbraith <mgalbraith@suse.de> - 2016-05-02 19:40 +0200
        Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-02 17:10 +0200
          Re: sched: tweak select_idle_sibling to look for idle threads Ingo Molnar <mingo@kernel.org> - 2016-05-02 18:10 +0200
            Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-03 13:40 +0200
              Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-03 20:30 +0200
        Re: sched: tweak select_idle_sibling to look for idle threads Peter Zijlstra <peterz@infradead.org> - 2016-05-02 17:20 +0200

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