linux-kernel - Re: [PATCH] sched/fair: Fix forked task check in vruntime

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Date: Mon, 1 Apr 2024 11:09:04 -0700
From: John Stultz <jstultz@...gle.com>
To: Honglei Wang <jameshongleiwang@....com>
Cc: "mingyang.cui" <mingyang.cui@...izon.ai>, mingo@...hat.com, peterz@...radead.org, 
	juri.lelli@...hat.com, vincent.guittot@...aro.org, dietmar.eggemann@....com, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, 
	linux-kernel@...r.kernel.org, kernel-team@...roid.com, stable@...r.kernel.org, 
	tkjos@...gle.com, pjt@...gle.com, quentin.perret@....com, 
	Patrick.Bellasi@....com, Chris.Redpath@....com, Morten.Rasmussen@....com, 
	joaodias@...gle.com
Subject: Re: [PATCH] sched/fair: Fix forked task check in vruntime_normalized

On Mon, Apr 1, 2024 at 8:20 AM Honglei Wang <jameshongleiwang@....com> wrote:
> On 2024/3/28 14:27, mingyang.cui wrote:
> > When rt_mutex_setprio changes a task's scheduling class to RT,
> > sometimes the task's vruntime is not updated correctly upon
> > return to the fair class.
> > Specifically, the following is being observed:
> > - task has just been created and running for a short time
> > - task sleep while still in the fair class
> > - task is boosted to RT via rt_mutex_setprio, which changes
> >    the task to RT and calls check_class_changed.
> > - check_class_changed leads to detach_task_cfs_rq, at which point
> >    the vruntime_normalized check sees that the task's sum_exec_runtime
> >    is zero, which results in skipping the subtraction of the
> >    rq's min_vruntime from the task's vruntime
>
> Did you do the test on the latest tree? vruntime_normalized was removed
> by e8f331bcc2 (sched/smp: Use lag to simplify cross-runqueue placement).

Indeed (I was looking at an older tree last week and missed it was
removed as well). Though something like this change probably will be
needed for the -stable trees?

thanks
-john