From: Mel Gorman <mgorman@techsingularity.net>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: peterz@infradead.org, 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: Fri, 23 Jul 2021 17:21:37 +0100 [thread overview]
Message-ID: <20210723162137.GY3809@techsingularity.net> (raw)
In-Reply-To: <ddb81bc9-1429-c392-adac-736e23977c84@de.ibm.com>
On Fri, Jul 23, 2021 at 02:36:21PM +0200, Christian Borntraeger wrote:
> > sched: Do not select highest priority task to run if it should be skipped
> >
> > <SNIP>
> >
> > 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.
> > */
> >
>
> I do see a reduction in ignored yields, but from a performance aspect for my
> testcases this patch does not provide a benefit, while the the simple
> curr->vruntime += sysctl_sched_min_granularity;
> does.
I'm still not a fan because vruntime gets distorted. From the docs
Small detail: on "ideal" hardware, at any time all tasks would have the same
p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
would ever get "out of balance" from the "ideal" share of CPU time
If yield_to impacts this "ideal share" then it could have other
consequences.
I think your patch may be performing better in your test case because every
"wrong" task selected that is not the yield_to target gets penalised and
so the yield_to target gets pushed up the list.
> I still think that your approach is probably the cleaner one, any chance to improve this
> somehow?
>
Potentially. The patch was a bit off because while it noticed that skip
was not being obeyed, the fix was clumsy and isolated. The current flow is
1. pick se == left as the candidate
2. try pick a different se if the "ideal" candidate is a skip candidate
3. Ignore the se update if next or last are set
Step 3 looks off because it ignores skip if next or last buddies are set
and I don't think that was intended. Can you try this?
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 44c452072a1b..d56f7772a607 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4522,12 +4522,12 @@ 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 && wakeup_preempt_entity(cfs_rq->next, se) < 1) {
/*
* Someone really wants this to run. If it's not unfair, run it.
*/
se = cfs_rq->next;
- } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
+ } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, se) < 1) {
/*
* Prefer last buddy, try to return the CPU to a preempted task.
*/
next prev parent reply other threads:[~2021-07-23 16:21 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 [this message]
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
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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