linux-kernel - AW: Crash in fair scheduler

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Date:   Tue, 3 Dec 2019 15:57:00 +0000
From:   "Schmid, Carsten" <Carsten_Schmid@...tor.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: AW: Crash in fair scheduler

> On 03/12/2019 12:09, Valentin Schneider wrote:
> > On 03/12/2019 10:40, Dietmar Eggemann wrote:
> >> On 03/12/2019 11:30, Valentin Schneider wrote:
> >>> On 03/12/2019 09:11, Schmid, Carsten wrote:
> >>
> >> [...]
> 
> I can't reproduce it on Arm64 Juno running 4.14.86. I suppose that there
> is no extra reproducer testcase since the issue happened with
> prev->sched_class eq. &idle_sched_class [prev eq. swapper/X 0] in the
> simple path of pick_next_task_fair().
> 
> I'm running with CONFIG_SCHED_AUTOGROUP=y and
> CONFIG_FAIR_GROUP_SCHED=y
> some taskgroup related tests for hours now. So the sched_entity (se) can
> be a task, an autogroup or a taskgroup in the simple path. pref is
> either swapper/X or migration/X.

We have the same kernel config settings.
However, as i stated in the analysis, we had
prev->sched_class ne. &fair_sched_class
and, unfortunately, no reproducer.

Looks like we need to find out why rb_leftmost is 0x0/NULL.