linux-kernel - Re: [tip: sched/core] sched/eevdf: More PELT vs DELAYED

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Message-ID: <ee2feb3b-0a1f-4276-b6fd-f36014654cbf@amd.com>
Date: Wed, 27 Nov 2024 09:47:54 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: "Peter Zijlstra (Intel)" <peterz@...radead.org>
CC: Dietmar Eggemann <dietmar.eggemann@....com>, Vincent Guittot
	<vincent.guittot@...aro.org>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, <linux-tip-commits@...r.kernel.org>
Subject: Re: [tip: sched/core] sched/eevdf: More PELT vs DELAYED_DEQUEUE

Hello Peter,

On 9/10/2024 1:39 PM, tip-bot2 for Peter Zijlstra wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     2e05f6c71d36f8ae1410a1cf3f12848cc17916e9
> Gitweb:        https://git.kernel.org/tip/2e05f6c71d36f8ae1410a1cf3f12848cc17916e9
> Author:        Peter Zijlstra <peterz@...radead.org>
> AuthorDate:    Fri, 06 Sep 2024 12:45:25 +02:00
> Committer:     Peter Zijlstra <peterz@...radead.org>
> CommitterDate: Tue, 10 Sep 2024 09:51:15 +02:00
> 
> sched/eevdf: More PELT vs DELAYED_DEQUEUE
> 
> Vincent and Dietmar noted that while commit fc1892becd56 fixes the
> entity runnable stats, it does not adjust the cfs_rq runnable stats,
> which are based off of h_nr_running.
> 
> Track h_nr_delayed such that we can discount those and adjust the
> signal.
> 
> Fixes: fc1892becd56 ("sched/eevdf: Fixup PELT vs DELAYED_DEQUEUE")
> Reported-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Reported-by: Vincent Guittot <vincent.guittot@...aro.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20240906104525.GG4928@noisy.programming.kicks-ass.net

I've been testing this fix for a while now to see if it helps the
regressions reported around EEVDF complete. The issue with negative
"h_nr_delayed" reported by Luis previously seem to have been fixed as a
result of commit 75b6499024a6 ("sched/fair: Properly deactivate
sched_delayed task upon class change")

I've been running stress-ng for a while and haven't seen any cases of
negative "h_nr_delayed". I'd also added the following WARN_ON() to see
if there are any delayed tasks on the cfs_rq before switching to idle in
some of my previous experiments and I did not see any splat during my
benchmark runs.

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 621696269584..c19a31fa46c9 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -457,6 +457,9 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
  
  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
  {
+	/* All delayed tasks must be picked off before switching to idle */
+	SCHED_WARN_ON(rq->cfs.h_nr_delayed);
+
  	update_idle_core(rq);
  	scx_update_idle(rq, true);
  	schedstat_inc(rq->sched_goidle);
--

If you are including this back, feel free to add:

Tested-by: K Prateek Nayak <kprateek.nayak@....com>

> [..snip..]

-- 
Thanks and Regards,
Prateek