linux-kernel - Re: [PATCH 18/19] sched: prctl() core-scheduling interface

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Message-ID: <20210817231734.GA4449@asgard.redhat.com>
Date:   Wed, 18 Aug 2021 01:17:34 +0200
From:   Eugene Syromiatnikov <esyr@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     joel@...lfernandes.org, chris.hyser@...cle.com, joshdon@...gle.com,
        mingo@...nel.org, vincent.guittot@...aro.org,
        valentin.schneider@....com, mgorman@...e.de,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Christian Brauner <christian.brauner@...ntu.com>, ldv@...ace.io
Subject: Re: [PATCH 18/19] sched: prctl() core-scheduling interface

On Tue, Aug 17, 2021 at 05:52:43PM +0200, Peter Zijlstra wrote:
> Urgh... lemme guess, your HP BIOS is funny and reports more possible
> CPUs than you actually have resulting in cpu_possible_mask !=
> cpu_online_mask. Alternatively, you booted with nr_cpus= or something
> daft like that.

Yep, it seems to be the case:

    # cat /sys/devices/system/cpu/possible
    0-7
    # cat /sys/devices/system/cpu/online
    0-3

> 
> That code does for_each_possible_cpus(i) { rq_lock_irq(cpu_rq(i)); },
> which, because of core-sched, needs rq->core set-up, but because these
> CPUs have never been online, that's not done and *BOOM*.
> 
> Or something like that.. I'll try and have a look tomorrow, I'm in dire
> need of sleep.
>