linux-kernel - Re: [PATCH 3/3] sched/core: Indicate a sched_delayed task was migrated before wakeup

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Message-ID: <ae7bd73b-63d8-941b-ba7c-5c857a013346@amd.com>
Date: Fri, 11 Oct 2024 09:01:03 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Johannes Weiner <hannes@...xchg.org>, Peter Zijlstra
	<peterz@...radead.org>
CC: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>, Suren Baghdasaryan
	<surenb@...gle.com>, <linux-kernel@...r.kernel.org>, Dietmar Eggemann
	<dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall
	<bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Valentin Schneider
	<vschneid@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Klaus Kudielka
	<klaus.kudielka@...il.com>, Chris Bainbridge <chris.bainbridge@...il.com>,
	"Linux regression tracking (Thorsten Leemhuis)" <regressions@...mhuis.info>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>, Youssef Esmat
	<youssefesmat@...gle.com>, Paul Menzel <pmenzel@...gen.mpg.de>, "Bert
 Karwatzki" <spasswolf@....de>, <regressions@...ts.linux.dev>
Subject: Re: [PATCH 3/3] sched/core: Indicate a sched_delayed task was
 migrated before wakeup

Hello Johannes,

On 10/11/2024 1:07 AM, Johannes Weiner wrote:
> On Thu, Oct 10, 2024 at 03:06:21PM +0200, Peter Zijlstra wrote:
>> On Thu, Oct 10, 2024 at 09:03:16AM -0400, Johannes Weiner wrote:
>>
>>> I'll try to come up with a suitable solution as well, please don't
>>> apply this one for now.
>>
>> I'll make sure it doesn't end up in tip as-is.
> 
> Thanks.
> 
> This would be a replacement patch for #2 and #3 that handles migration
> of delayed tasks. It's slightly more invasive on the psi callback
> side, but I think it keeps the sched core bits simpler. Thoughts?
> 
> ---
> 
>  From d72a665d7c7c7d9c806424f473d13452754471d3 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@...xchg.org>
> Date: Thu, 10 Oct 2024 14:37:43 -0400
> Subject: [PATCH] sched: psi: handle delayed-dequeue task migration
> 
> Since sched_delayed tasks remain queued even after blocking, the load
> balancer can migrate them between runqueues while PSI considers them
> to be asleep. As a result, it misreads the migration requeue followed
> by a wakeup as a double queue:
> 
>    psi: inconsistent task state! task=... cpu=... psi_flags=4 clear=. set=4
> 
> First, call psi_enqueue() after p->sched_class->enqueue_task(). A
> wakeup will clear p->se.sched_delayed while a migration will not, so
> psi can use that flag to tell them apart.
> 
> Then teach psi to migrate any "sleep" state when delayed-dequeue tasks
> are being migrated.
> 
> Delayed-dequeue tasks can be revived by ttwu_runnable(), which will
> call down with a new ENQUEUE_DELAYED. Instead of further complicating
> the wakeup conditional in enqueue_task(), identify migration contexts
> instead and default to wakeup handling for all other cases.
> 
> Debugged-by-and-original-fix-by: K Prateek Nayak <kprateek.nayak@....com>
> Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
> Closes: https://lore.kernel.org/lkml/20240830123458.3557-1-spasswolf@web.de/
> Closes: https://lore.kernel.org/all/cd67fbcd-d659-4822-bb90-7e8fbb40a856@molgen.mpg.de/
> Link: https://lore.kernel.org/lkml/f82def74-a64a-4a05-c8d4-4eeb3e03d0c0@amd.com/
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

This approach looks good to me as well! Thank you. I added this on top
of Patch 1 and I haven't seen any PSI splats after my stress test. Feel
free to add:

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

-- 
Thanks and Regards,
Prateek

> ---
>   kernel/sched/core.c  | 12 +++++------
>   kernel/sched/stats.h | 48 ++++++++++++++++++++++++++++++--------------
>   2 files changed, 39 insertions(+), 21 deletions(-)
> 
> [..snip..]