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

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Message-ID: <20250416075925.GB6580@noisy.programming.kicks-ass.net>
Date: Wed, 16 Apr 2025 09:59:25 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Chris Mason <clm@...a.com>
Cc: Rik van Riel <riel@...riel.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, stable@...r.kernel.org,
	Breno Leitao <leitao@...ian.org>
Subject: Re: [PATCH] sched/fair: Add null pointer check to pick_next_entity()

On Tue, Apr 15, 2025 at 12:07:05PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 14, 2025 at 11:38:15AM -0400, Chris Mason wrote:
> > 
> > 
> > On 4/14/25 5:08 AM, Peter Zijlstra wrote:
> > 
> > [ math and such ]
> > 
> > 
> > > The zero_vruntime patch I gave earlier should avoid this particular
> > > issue.
> > 
> > Here's a crash with the zero runtime patch. 
> 
> And indeed it doesn't have these massive (negative) avg_vruntime values.
> 
> > I'm trying to reproduce
> > this outside of prod so we can crank up the iteration speed a bit.
> 
> Thanks.
> 
> Could you add which pick went boom for the next dump?
> 
> 
> 
> I am however, slightly confused by this output format.
> 
> It looks like it dumps the cfs_rq the first time it encounters it,
> either through curr or through the tree.
> 
> So if I read this correct the root is something like:
> 
> > nr_running = 2
> > zero_vruntime = 19194347104893960
> > avg_vruntime = 6044054790
> > avg_load = 2
> > curr = {
> >   cgroup urgent
> >   vruntime = 24498183812106172
> >   weight = 3561684 => 3478
> > }
> > tasks_timeline = [
> >   {
> >     cgroup optional
> >     vruntime = 19194350126921355
> >     weight = 1168 => 2
> >   },
> > ]
> 
> group  19194347104893960
> curr   24498183812106172 3561684
> entity 19194350126921355 1168
> 
> But if I run those numbers, I get avg_load == 1, seeing how 1168/1024 =
> 1. But the thing says it should be 2.

N/m, late last night I remembered we have a max(2, ..) in there. So
yeah, your numbers seem right.