linux-kernel - Re: "Dying CPU not properly vacated" splat

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Message-ID: <xhsmhsfmfc7s5.mognet@vschneid.remote.csb>
Date:   Tue, 02 Aug 2022 10:30:02 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.org, 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
Subject: Re: "Dying CPU not properly vacated" splat

On 05/07/22 10:23, Paul E. McKenney wrote:
> The second of these occurred near shutdown, but the first was quite some
> time before shutdown.  In case that makes a difference.
>
> I have not seen this since.
>
> Any other diagnostics I should add?
>

Sorry, I let this get buried to the bottom of my inbox :(

I've had another look at rcutorture.c but just like for
rcu_torture_reader(), I don't see any obvious culprit (no
kthread_set_per_cpu() usage)).

One thing I think would help is a scheduling trace (say sched_switch,
sched_wakeup and cpuhp*, combined with ftrace_dump_on_oops + panic_on_warn
?) - that should at least tell us if the issue is in the wakeup placement
(if the task gets placed on a dying CPU *after* CPUHP_AP_ACTIVE), or in the
balance_push() mechanism (the task was *already* on the CPU when it started
dying and never moved away).

Neither make sense to me, but it has to be somewhere in there...

>                                                       Thanx, Paul