linux-kernel - Re: [patch 0/3] lib/percpu_counter, cpu/hotplug: Cure the cpu_dying

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Date:   Wed, 03 May 2023 12:50:07 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Dave Chinner <dchinner@...hat.com>,
        Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ye Bin <yebin10@...wei.com>, linux-mm@...ck.org
Subject: Re: [patch 0/3] lib/percpu_counter, cpu/hotplug: Cure the
 cpu_dying_mask woes

On 14/04/23 18:30, Thomas Gleixner wrote:
> Hi!
>
> The cpu_dying_mask is not only undocumented but also to some extent a
> misnomer. It's purpose is to capture the last direction of a cpu_up() or
> cpu_down() operation taking eventual rollback operations into account.
>
> cpu_dying mask is not really useful for general consumption. The
> cpu_dying_mask bits are sticky even after cpu_up() or cpu_down() completes.
>
> A recent fix to plug a race in the per CPU counter code picked
> cpu_dying_mask to cure it. Unfortunately this does not work as the author
> probably expected and the behaviour of cpu_dying_mask is not easy to change
> without breaking the only other and initial user, the scheduler.
>
> This series addresses this by:
>
>    1) Reworking the per CPU counter hotplug mechanism so the race is fully
>       plugged without using cpu_dying_mask
>
>    2) Replacing the cpu_dying_mask logic with hotplug core internal state
>       which is exposed to the scheduler with a properly documented
>       function.
>

For patches 2-3:

Reviewed-by: Valentin Schneider <vschneid@...hat.com>