linux-kernel - Re: Null pointer crash at find_idlest

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Message-ID: <db68d480-dc1e-04d5-1a20-966b5eddb26a@arm.com>
Date:   Wed, 4 Dec 2019 14:06:59 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Qais Yousef <qais.yousef@....com>,
        John Stultz <john.stultz@...aro.org>,
        Quentin Perret <qperret@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Patrick Bellasi <Patrick.Bellasi@....com>,
        Ingo Molnar <mingo@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Null pointer crash at find_idlest_group on db845c w/ linus/master

On 04/12/2019 12:08, Vincent Guittot wrote:
>> Also; does it really have to involve an affinity "race"? AFAIU affinity
>> could have been changed a while back, but the waking CPU isn't allowed
>> so we skip the local_group (in simpler cases where each CPU is a group).
> 
> In fact, this will depend of the uninitialized values of local_sgs. I
> have been able to reproduce the situation where we skip local group
> but not to trigger the crash because the values already in the stack
> don't trigger the misfit comparison.
> 

One more thing, DB845 has a single DynamIQ cluster that is represented as a
flat hierarchy (unlike regular big.LITTLE, see
arch/arm64/boot/dts/qcom/sdm845.dtsi) so we'll just have a single MC level
with groups being individual CPUs, making the bug easier to reproduce
(than on regular big.LITTLE, that is). 

> I  wait for John feedback to confirm that this fix his problem and
> will send a clean version of the patch
>>
>>