linux-kernel - Re: [PATCH] sched/fair: Add null pointer check to pick_next

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Message-ID: <abffc286b637060f631925f9b373fad114d667d6.camel@surriel.com>
Date: Wed, 16 Apr 2025 10:19:42 -0400
From: Rik van Riel <riel@...riel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Chris Mason <clm@...a.com>, Pat Cody <pat@...cody.io>, mingo@...hat.com,
 	juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, 	rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com, 	linux-kernel@...r.kernel.org,
 patcody@...a.com, kernel-team@...a.com, Breno Leitao	 <leitao@...ian.org>
Subject: Re: [PATCH] sched/fair: Add null pointer check to pick_next_entity()

On Wed, 2025-04-16 at 14:44 +0200, Peter Zijlstra wrote:
> On Tue, Apr 15, 2025 at 10:02:35AM +0200, Peter Zijlstra wrote:
> > On Mon, Apr 14, 2025 at 03:57:42PM -0400, Rik van Riel wrote:
> > > On Wed, 2025-04-02 at 10:22 +0200, Peter Zijlstra wrote:
> > > > 
> > > > Please confirm what the reason for overflow is.
> > > > 
> > > Running a large enough sample size has its benefits.
> > > 
> > > We have hit 3 out of the 4 warnings below.
> > > 
> > > The only one we did not hit is the cfs_rq->avg_load != avg_load
> > > warning.
> > 
> > Fair enough, that one really isn't hard.
> > 
> > > Most of the time we seem to hit the warnings from the
> > > code that removes tasks from the runqueue, 
> > 
> > *blink*..
> 
> Which warning is getting hit on removal? The avg_vruntime mismatch?
> 
> Also, which removal path? schedule()'s block path, or migration like?

The most common warning by far, hitting
about 90% of the time we hit anything
in avg_vruntime_validate is the
WARN_ON_ONCE(cfs_rq->avg_vruntime != vruntime)

The most common code path to getting there,
covering about 85% of the cases:

avg_vruntime_validate
avg_vruntime_sub
__dequeue_entity
set_next_entity
pick_task_fair
pick_next_task_fair
__pick_next_task
pick_next_task
__schedule
schedule


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