linux-kernel - Re: [REGRESSION] Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

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Message-ID: <20241009180719.GD6937@cmpxchg.org>
Date: Wed, 9 Oct 2024 14:07:19 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Klaus Kudielka <klaus.kudielka@...il.com>,
	Chris Bainbridge <chris.bainbridge@...il.com>,
	linux-kernel@...r.kernel.org, bsegall@...gle.com,
	dietmar.eggemann@....com, efault@....de, juri.lelli@...hat.com,
	mgorman@...e.de, mingo@...hat.com, rostedt@...dmis.org,
	tglx@...utronix.de, vincent.guittot@...aro.org, vschneid@...hat.com,
	wuyun.abel@...edance.com, youssefesmat@...omium.org,
	spasswolf@....de, regressions@...ts.linux.dev,
	"Linux regression tracking (Thorsten Leemhuis)" <regressions@...mhuis.info>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>
Subject: Re: [REGRESSION] Re: [PATCH 17/24] sched/fair: Implement delayed
 dequeue

On Tue, Oct 08, 2024 at 09:54:52PM +0530, K Prateek Nayak wrote:
> From 2e15180e18b51e9a2bc0d7050e915a70d2673a06 Mon Sep 17 00:00:00 2001
> From: K Prateek Nayak <kprateek.nayak@....com>
> Date: Fri, 4 Oct 2024 15:24:35 +0000
> Subject: [RFC PATCH] sched/psi: Fixup PSI accounting with DELAY_DEQUEUE
> 
> After the merge of DELAY_DEQUEUE, "psi: inconsistent task state: warning
> were seen early into the boot. The crux of the matter is the fact that
> when a task is delayed, and the delayed task is then migrated, the
> wakeup context may not have any idea that the task was moved from its
> previous runqueue. This is the same reason psi_enqueue() considers
> only ...
> 
>      (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED)
> 
> ... as a wakeup. In case of a wakeup with migration, PSI forgoes
> clearing the TSK_IOWAIT flag which seems to be the issue I encountered
> in my splat previously.
> 
> With that said, the below diff, based on Peter's original approach
> currently seems to work for me in the sense that I have not seen the
> inconsistent state warning for a while now with my stress test.
> 
> Two key points of the approach are:
> 
> o It uses "p->migration_flags" to indicate a delayed entity has
>    migrated to another runqueue and convey the same during psi_enqueue().
> 
> o It adds ENQUEUE_WAKEUP flag alongside ENQUEUE_DELAYED for
>    enqueue_task() in ttwu_runnable() since psi_enqueue() needs to know of
>    a wakeup without migration to clear the TSK_IOWAIT flag it would have
>    set during psi_task_switch() for blocking task and going down the
>    stack for enqueue_task_fair(), there seem to be no other observer of
>    the ENQUEUE_WAKEUP flag other than psi_enqueue() in the requeue path.
> 
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>

Tested-by: Johannes Weiner <hannes@...xchg.org>

It fixes the warning and bogus pressure values after stressing it for
an hour or so with tons of cpu contention and cgroup movements.