From: Peter Zijlstra <peterz@infradead.org>
To: Chen Yu <yu.c.chen@intel.com>
Cc: mingo@redhat.com, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
bristot@redhat.com, vschneid@redhat.com,
linux-kernel@vger.kernel.org, kprateek.nayak@amd.com,
wuyun.abel@bytedance.com, tglx@linutronix.de, efault@gmx.de,
yu.chen.surf@gmail.com
Subject: Re: [RFC][PATCH 08/10] sched/fair: Implement delayed dequeue
Date: Mon, 8 Apr 2024 11:06:39 +0200 [thread overview]
Message-ID: <20240408090639.GD21904@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <ZhEUjX1Nw0y2eJ1o@chenyu5-mobl2>
On Sat, Apr 06, 2024 at 05:23:25PM +0800, Chen Yu wrote:
> The 99th wakeup latency increases a little bit, and should be in the acceptible
> range(25 -> 31 us).
Ah, my test runs haven't been stable enough to observe that.
> Meanwhile the throughput increases accordingly. Here are
> the possible reason I can think of:
>
> 1. wakeup latency: The time to find an eligible entity in the tree
> during wakeup might take longer - if there are more delayed-dequeue
> tasks in the tree.
Another possible cause might be that previously a schedule() would be
1 dequeue, 1 pick.
But now it can be much more variable, a pick can basically do N dequeues
and N+1 picks.
So not only do we do more picks, but if you're focussed on worst case
latency, it goes up, because we can do multiple dequeues for a single
pick.
If we find this to really be a problem, I had some half baked ideas to
fix it, but it added significant complexity, so keep it simple until
need proves we need more etc.
> 2. throughput: Inhibit task dequeue can decrease the ratio to touch the
> task group's load_avg: dequeue_entity()-> { update_load_avg(), update_cfs_group()),
> which reduces the cache contention among CPUs, and improves throughput.
Ah, yes, there's that.
> > + } else {
> > + bool sleep = flags & DEQUEUE_SLEEP;
> > +
> > + SCHED_WARN_ON(sleep && se->sched_delayed);
> > + update_curr(cfs_rq);
> > +
> > + if (sched_feat(DELAY_DEQUEUE) && sleep &&
> > + !entity_eligible(cfs_rq, se)) {
>
> Regarding the elibigle check, it was found that there could be an overflow
> issue, and it brings false negative of entity_eligible(), which was described here:
> https://lore.kernel.org/lkml/20240226082349.302363-1-yu.c.chen@intel.com/
> and also reported on another machine
> https://lore.kernel.org/lkml/ZeCo7STWxq+oyN2U@gmail.com/
> I don't have good idea to avoid that overflow properly, while I'm trying to
> reproduce it locally, do you have any guidance on how to address it?
I have not yet seen those, let me go stare at them now. Thanks!
next prev parent reply other threads:[~2024-04-08 9:06 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-05 10:27 [RFC][PATCH 00/10] sched/fair: Complete EEVDF Peter Zijlstra
2024-04-05 10:27 ` [RFC][PATCH 01/10] sched/eevdf: Add feature comments Peter Zijlstra
2024-04-05 10:27 ` [RFC][PATCH 02/10] sched/eevdf: Remove min_vruntime_copy Peter Zijlstra
2024-04-05 10:27 ` [RFC][PATCH 03/10] sched/fair: Cleanup pick_task_fair() vs throttle Peter Zijlstra
2024-04-05 21:11 ` Benjamin Segall
2024-04-05 10:27 ` [RFC][PATCH 04/10] sched/fair: Cleanup pick_task_fair()s curr Peter Zijlstra
2024-04-05 10:27 ` [RFC][PATCH 05/10] sched/fair: Unify pick_{,next_}_task_fair() Peter Zijlstra
2024-04-06 2:20 ` Mike Galbraith
2024-04-05 10:28 ` [RFC][PATCH 06/10] sched: Allow sched_class::dequeue_task() to fail Peter Zijlstra
2024-04-05 10:28 ` [RFC][PATCH 07/10] sched/fair: Re-organize dequeue_task_fair() Peter Zijlstra
2024-04-05 10:28 ` [RFC][PATCH 08/10] sched/fair: Implement delayed dequeue Peter Zijlstra
2024-04-06 9:23 ` Chen Yu
2024-04-08 9:06 ` Peter Zijlstra [this message]
2024-04-11 1:32 ` Yan-Jie Wang
2024-04-25 10:25 ` Peter Zijlstra
2024-04-12 10:42 ` K Prateek Nayak
2024-04-15 10:56 ` Mike Galbraith
2024-04-16 3:18 ` K Prateek Nayak
2024-04-16 5:36 ` Mike Galbraith
2024-04-18 16:24 ` Mike Galbraith
2024-04-18 17:08 ` K Prateek Nayak
2024-04-24 15:20 ` Peter Zijlstra
2024-04-25 11:28 ` Peter Zijlstra
2024-04-26 10:56 ` Peter Zijlstra
2024-04-26 11:16 ` Peter Zijlstra
2024-04-26 16:03 ` Mike Galbraith
2024-04-27 6:42 ` Mike Galbraith
2024-04-28 16:32 ` Mike Galbraith
2024-04-29 12:14 ` Peter Zijlstra
2024-04-15 17:07 ` Luis Machado
2024-04-24 15:15 ` Luis Machado
2024-04-25 10:42 ` Peter Zijlstra
2024-04-25 11:49 ` Peter Zijlstra
2024-04-26 9:32 ` Peter Zijlstra
2024-04-26 9:36 ` Peter Zijlstra
2024-04-26 10:16 ` Luis Machado
2024-04-29 14:33 ` Luis Machado
2024-05-02 10:26 ` Luis Machado
2024-05-10 14:49 ` Luis Machado
2024-05-15 9:36 ` Peter Zijlstra
2024-05-15 11:48 ` Peter Zijlstra
2024-05-15 18:03 ` Mike Galbraith
2024-04-26 10:15 ` Luis Machado
2024-04-20 5:57 ` Mike Galbraith
2024-04-22 13:13 ` Tobias Huschle
2024-04-05 10:28 ` [RFC][PATCH 09/10] sched/eevdf: Allow shorter slices to wakeup-preempt Peter Zijlstra
2024-04-05 10:28 ` [RFC][PATCH 10/10] sched/eevdf: Use sched_attr::sched_runtime to set request/slice suggestion Peter Zijlstra
2024-04-06 8:16 ` Hillf Danton
2024-05-07 5:34 ` Mike Galbraith
2024-05-15 10:13 ` Peter Zijlstra
2024-05-07 15:15 ` Chen Yu
2024-05-08 13:52 ` Mike Galbraith
2024-05-09 3:48 ` Chen Yu
2024-05-09 5:00 ` Mike Galbraith
2024-05-13 4:07 ` K Prateek Nayak
2024-05-14 9:18 ` Chen Yu
2024-05-14 15:23 ` K Prateek Nayak
2024-05-14 16:15 ` Chen Yu
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