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

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Message-ID: <xhsmha5gwome6.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 28 Aug 2024 14:35:45 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: paulmck@...nel.org
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,
 Chen Yu <yu.c.chen@...el.com>
Subject: Re: [BUG almost bisected] Splat in dequeue_rt_stack() and build error

On 27/08/24 13:36, Paul E. McKenney wrote:
> On Tue, Aug 27, 2024 at 10:30:24PM +0200, Valentin Schneider wrote:
>> On 27/08/24 11:35, Paul E. McKenney wrote:
>> > On Tue, Aug 27, 2024 at 10:33:13AM -0700, Paul E. McKenney wrote:
>> >> On Tue, Aug 27, 2024 at 05:41:52PM +0200, Valentin Schneider wrote:
>> >> > I've taken tip/sched/core and shuffled hunks around; I didn't re-order any
>> >> > commit. I've also taken out the dequeue from switched_from_fair() and put
>> >> > it at the very top of the branch which should hopefully help bisection.
>> >> >
>> >> > The final delta between that branch and tip/sched/core is empty, so it
>> >> > really is just shuffling inbetween commits.
>> >> >
>> >> > Please find the branch at:
>> >> >
>> >> > https://gitlab.com/vschneid/linux.git -b mainline/sched/eevdf-complete-builderr
>> >> >
>> >> > I'll go stare at the BUG itself now.
>> >>
>> >> Thank you!
>> >>
>> >> I have fired up tests on the "BROKEN?" commit.  If that fails, I will
>> >> try its predecessor, and if that fails, I wlll bisect from e28b5f8bda01
>> >> ("sched/fair: Assert {set_next,put_prev}_entity() are properly balanced"),
>> >> which has stood up to heavy hammering in earlier testing.
>> >
>> > And of 50 runs of TREE03 on the "BROKEN?" commit resulted in 32 failures.
>> > Of these, 29 were the dequeue_rt_stack() failure.  Two more were RCU
>> > CPU stall warnings, and the last one was an oddball "kernel BUG at
>> > kernel/sched/rt.c:1714" followed by an equally oddball "Oops: invalid
>> > opcode: 0000 [#1] PREEMPT SMP PTI".
>> >
>> > Just to be specific, this is commit:
>> >
>> > df8fe34bfa36 ("BROKEN? sched/fair: Dequeue sched_delayed tasks when switching from fair")
>> >
>> > This commit's predecessor is this commit:
>> >
>> > 2f888533d073 ("sched/eevdf: Propagate min_slice up the cgroup hierarchy")
>> >
>> > This predecessor commit passes 50 runs of TREE03 with no failures.
>> >
>> > So that addition of that dequeue_task() call to the switched_from_fair()
>> > function is looking quite suspicious to me.  ;-)
>> >
>> >                                                       Thanx, Paul
>>
>> Thanks for the testing!
>>
>> The WARN_ON_ONCE(!rt_se->on_list); hit in __dequeue_rt_entity() feels like
>> a put_prev/set_next kind of issue...
>>
>> So far I'd assumed a ->sched_delayed task can't be current during
>> switched_from_fair(), I got confused because it's Mond^CCC Tuesday, but I
>> think that still holds: we can't get a balance_dl() or balance_rt() to drop
>> the RQ lock because prev would be fair, and we can't get a
>> newidle_balance() with a ->sched_delayed task because we'd have
>> sched_fair_runnable() := true.
>>
>> I'll pick this back up tomorrow, this is a task that requires either
>> caffeine or booze and it's too late for either.
>
> Thank you for chasing this, and get some sleep!  This one is of course
> annoying, but it is not (yet) an emergency.  I look forward to seeing
> what you come up with.
>
> Also, I would of course be happy to apply debug patches.
>
>                                                       Thanx, Paul

Chen Yu made me realize [1] that dequeue_task() really isn't enough; the
dequeue_task() in e.g. __sched_setscheduler() won't have DEQUEUE_DELAYED,
so stuff will just be left on the CFS tree.

Worse, what we need here is the __block_task() like we have at the end of
dequeue_entities(), otherwise p stays ->on_rq and that's borked - AFAICT
that explains the splat you're getting, because affine_move_task() ends up
doing a move_queued_task() for what really is a dequeued task.

I unfortunately couldn't reproduce the issue locally using your TREE03
invocation. I've pushed a new patch on top of my branch, would you mind
giving it a spin? It's a bit sketchy but should at least be going in the
right direction...

[1]: http://lore.kernel.org/r/Zs2d2aaC/zSyR94v@chenyu5-mobl2