linux-kernel - Re: [BUG almost bisected] Splat in dequeue_rt

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Message-ID: <b1824f4a-f5cc-4011-876f-8a73cf752067@paulmck-laptop>
Date: Mon, 26 Aug 2024 09:31:07 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Valentin Schneider <vschneid@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	sfr@...b.auug.org.au, linux-next@...r.kernel.org,
	kernel-team@...a.com
Subject: Re: [BUG almost bisected] Splat in dequeue_rt_stack() and build error

On Mon, Aug 26, 2024 at 01:44:35PM +0200, Valentin Schneider wrote:
> On 23/08/24 09:47, Peter Zijlstra wrote:
> > On Wed, Aug 21, 2024 at 02:57:16PM -0700, Paul E. McKenney wrote:
> >
> >> 2e0199df252a ("sched/fair: Prepare exit/cleanup paths for delayed_dequeue")
> >>
> >> The preceding commit is very reliable.
> >>
> >> Only instead of (or maybe as well as?) introducing the dequeue_rt_stack()
> >> bug, the 2e0199df252a commit introduced a build bug:
> >>
> >> ------------------------------------------------------------------------
> >>
> >> In file included from kernel/sched/fair.c:54:
> >> kernel/sched/fair.c: In function ‘switched_from_fair’:
> >> kernel/sched/sched.h:2154:58: error: ‘__SCHED_FEAT_DELAY_ZERO’ undeclared (first use in this function); did you mean ‘__SCHED_FEAT_LATENCY_WARN’?
> >>  2154 | #define sched_feat(x) !!(sysctl_sched_features & (1UL << __SCHED_FEAT_##x))
> >>       |                                                          ^~~~~~~~~~~~~
> >> kernel/sched/fair.c:12878:21: note: in expansion of macro ‘sched_feat’
> >> 12878 |                 if (sched_feat(DELAY_ZERO) && p->se.vlag > 0)
> >>       |                     ^~~~~~~~~~
> >> kernel/sched/sched.h:2154:58: note: each undeclared identifier is reported only once for each function it appears in
> >>  2154 | #define sched_feat(x) !!(sysctl_sched_features & (1UL << __SCHED_FEAT_##x))
> >>       |                                                          ^~~~~~~~~~~~~
> >> kernel/sched/fair.c:12878:21: note: in expansion of macro ‘sched_feat’
> >> 12878 |                 if (sched_feat(DELAY_ZERO) && p->se.vlag > 0)
> >>       |                     ^~~~~~~~~~
> >>
> >
> > Oh gawd, last minute back-merges :/
> >
> > Does the below help any? That's more or less what it was before Valentin
> > asked me why it was weird like that :-)
> 
> Woops...

On the other hand, removing that dequeue_task() makes next-20240823
pass light testing.

I have to ask...

Does it make sense for Valentin to rearrange those commits to fix
the two build bugs and remove that dequeue_task(), all in the name of
bisectability.  Or is there something subtle here so that only Peter
can do this work, shoulder and all?

							Thanx, Paul