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From: Mel Gorman <mgorman@techsingularity.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
	bristot@redhat.com, bsegall@google.com, dietmar.eggemann@arm.com,
	joshdon@google.com, juri.lelli@redhat.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
	linux@rasmusvillemoes.dk, mgorman@suse.de, mingo@kernel.org,
	rostedt@goodmis.org, valentin.schneider@arm.com,
	vincent.guittot@linaro.org
Subject: Re: [PATCH 1/1] sched/fair: improve yield_to vs fairness
Date: Tue, 27 Jul 2021 15:31:28 +0100	[thread overview]
Message-ID: <20210727143128.GA3809@techsingularity.net> (raw)
In-Reply-To: <YQALDHw7Cr+vbeqN@hirez.programming.kicks-ass.net>

On Tue, Jul 27, 2021 at 03:33:00PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 23, 2021 at 10:35:23AM +0100, Mel Gorman wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 44c452072a1b..ddc0212d520f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4522,7 +4522,8 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> >  			se = second;
> >  	}
> >  
> > -	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
> > +	if (cfs_rq->next &&
> > +	    (cfs_rq->skip == left || wakeup_preempt_entity(cfs_rq->next, left) < 1)) {
> >  		/*
> >  		 * Someone really wants this to run. If it's not unfair, run it.
> >  		 */
> 
> With a little more context this function reads like:
> 
> 	se = left;
> 
> 	if (cfs_rq->skip && cfs_rq->skip == se) {
> 		...
> +		if (cfs_rq->next && (cfs_rq->skip == left || ...))
> 
> If '...' doesn't change @left (afaict it doesn't), then your change (+)
> is equivalent to '&& true', or am I reading things wrong?

You're not reading it wrong although the patch is clumsy and may introduce
unfairness that gets incrementally worse if there was repeated yields to
the same task. A second patch was posted that does

-       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, se) < 1) {

i.e. if the skip hint picks a second alternative then next or last buddies
should be compared to the second alternative and not "left". It doesn't
help indicating that the skip hint is not obeyed because "second" failed
the entity_before() or wakeup_preempt_entity() checks. I'm waiting on a
trace to see which check dominates.

That said, I'm still undecided on how to approach this. None of the
proposed patches on their own helps but the options are

1. Strictly obey the next buddy if the skip hint is the same se as left
   (first patch which I'm not very happy with even if it helped the
   test case)

2. My second patch which compares next/last with "second" if the skip
   hint skips "left". This may be a sensible starting point no matter
   what

3. Relaxing how "second" is selected if next or last buddies are set

4. vruntime tricks even if it punishes fairness for the task yielding
   the CPU. The advantage of this approach is if there are multiple tasks
   ahead of the task being yielded to then yield_to task will become
   "left" very quickly regardless of any buddy-related hints.

I don't know what "3" would look like yet, it might be very fragile but
lets see what the tracing says. Otherwise, testing 2+4 might be worthwhile
to see if the combination helps Christian's test case when the cpu cgroup
is involved.

-- 
Mel Gorman
SUSE Labs

  reply	other threads:[~2021-07-27 14:31 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-12 10:14 [PATCH v2 0/9] sched: Clean up SCHED_DEBUG Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 1/9] sched/numa: Allow runtime enabling/disabling of NUMA balance without SCHED_DEBUG Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 2/9] sched: Remove sched_schedstats sysctl out from under SCHED_DEBUG Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 3/9] sched: Dont make LATENCYTOP select SCHED_DEBUG Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] sched: Don't " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 4/9] sched: Move SCHED_DEBUG sysctl to debugfs Peter Zijlstra
2021-04-15 16:29   ` [PATCH] sched/debug: Rename the sched_debug parameter to sched_debug_verbose Peter Zijlstra
2021-04-19 19:26     ` Josh Don
2021-04-16 15:53   ` [tip: sched/core] sched: Move SCHED_DEBUG sysctl to debugfs tip-bot2 for Peter Zijlstra
2021-04-27 14:59   ` Christian Borntraeger
2021-04-27 15:09     ` Steven Rostedt
2021-04-27 15:17       ` Christian Borntraeger
2021-04-28  8:47       ` Peter Zijlstra
2021-04-28  8:46     ` Peter Zijlstra
2021-04-28  8:54       ` Christian Borntraeger
2021-04-28  8:58         ` Christian Borntraeger
2021-04-28  9:25         ` Peter Zijlstra
2021-04-28  9:31           ` Christian Borntraeger
2021-04-28  9:42       ` Christian Borntraeger
2021-04-28 12:38         ` Peter Zijlstra
2021-04-28 14:49           ` Christian Borntraeger
2021-07-07 12:34           ` [PATCH 0/1] Improve yield (was: sched: Move SCHED_DEBUG sysctl to debugfs) Christian Borntraeger
2021-07-07 12:34             ` [PATCH 1/1] sched/fair: improve yield_to vs fairness Christian Borntraeger
2021-07-07 18:07               ` kernel test robot
2021-07-23  9:35               ` Mel Gorman
2021-07-23 12:36                 ` Christian Borntraeger
2021-07-23 16:21                   ` Mel Gorman
2021-07-26 18:41                     ` Christian Borntraeger
2021-07-26 19:32                       ` Mel Gorman
2021-07-27  6:59                         ` Christian Borntraeger
2021-07-27 18:57                       ` Benjamin Segall
2021-07-28 16:23                         ` Christian Borntraeger
2021-08-10  8:49                           ` Vincent Guittot
2021-07-27 13:29                     ` Peter Zijlstra
2021-07-27 13:33                 ` Peter Zijlstra
2021-07-27 14:31                   ` Mel Gorman [this message]
2021-04-12 10:14 ` [PATCH v2 5/9] sched,preempt: Move preempt_dynamic to debug.c Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 6/9] debugfs: Implement debugfs_create_str() Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 7/9] sched,debug: Convert sysctl sched_domains to debugfs Peter Zijlstra
2021-04-13 14:55   ` Valentin Schneider
2021-04-15  9:06     ` Peter Zijlstra
2021-04-15 12:16       ` Dietmar Eggemann
2021-04-15 12:34       ` Valentin Schneider
2021-04-15 13:02         ` Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 8/9] sched: Move /proc/sched_debug " Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 9/9] sched,fair: Alternative sched_slice() Peter Zijlstra
2021-04-12 10:26   ` Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra

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