linux-kernel - Re: [PATCH v3 4/5] sched/fair: Task based throttle time accounting

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Message-ID: <xhsmhqzx7h08x.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 19 Aug 2025 16:09:02 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Aaron Lu <ziqianlu@...edance.com>
Cc: Ben Segall <bsegall@...gle.com>, K Prateek Nayak
 <kprateek.nayak@....com>, Peter Zijlstra <peterz@...radead.org>, Chengming
 Zhou <chengming.zhou@...ux.dev>, Josh Don <joshdon@...gle.com>, Ingo
 Molnar <mingo@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
 Xi Wang <xii@...gle.com>, linux-kernel@...r.kernel.org, Juri Lelli
 <juri.lelli@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Mel Gorman <mgorman@...e.de>, Chuyi
 Zhou <zhouchuyi@...edance.com>, Jan Kiszka <jan.kiszka@...mens.com>,
 Florian Bezdeka <florian.bezdeka@...mens.com>, Songtang Liu
 <liusongtang@...edance.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v3 4/5] sched/fair: Task based throttle time accounting

On 19/08/25 17:34, Aaron Lu wrote:
> On Mon, Aug 18, 2025 at 04:57:27PM +0200, Valentin Schneider wrote:
>> On 15/07/25 15:16, Aaron Lu wrote:
>> Apologies if this has been discussed before.
>>
>> So the throttled time (as reported by cpu.stat.local) is now accounted as
>> the time from which the first task in the hierarchy gets effectively
>> throttled - IOW the first time a task in a throttled hierarchy reaches
>> resume_user_mode_work() - as opposed to as soon as the hierarchy runs out
>> of quota.
>
> Right.
>
>>
>> The gap between the two shouldn't be much, but that should at the very
>> least be highlighted in the changelog.
>>
>
> Got it, does the below added words make this clear?
>

Yes, thank you. Small corrections below.

>     With task based throttle model, the previous way to check cfs_rq's
>     nr_queued to decide if throttled time should be accounted doesn't work
>     as expected, e.g. when a cfs_rq which has a single task is throttled,
>     that task could later block in kernel mode instead of being dequeued on
>     limbo list and account this as throttled time is not accurate.
                     ^^^^^^
                     accounting
>
>     Rework throttle time accounting for a cfs_rq as follows:
>     - start accounting when the first task gets throttled in its hierarchy;
>     - stop accounting on unthrottle.
>
>     Note that there will be a time gap between when a cfs_rq is throttled
>     and when a task in its hierarchy is actually throttled. This accounting
>     mechanism only started accounting in the latter case.
                     ^^^^^^
                     starts