linux-kernel - Re: [PATCH V3] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus

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Message-ID: <CAFj5m9LGi4zxvDYqTvSHYh=Ra3YHUMe3nePZqJsMfEXttbrgMA@mail.gmail.com>
Date:   Mon, 28 Aug 2023 08:58:28 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>, Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Keith Busch <kbusch@...nel.org>,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        Yi Zhang <yi.zhang@...hat.com>,
        Guangwu Zhang <guazhang@...hat.com>,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH V3] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly

On Fri, Aug 18, 2023 at 10:01 PM Ming Lei <ming.lei@...hat.com> wrote:
>
> group_cpus_evenly() could be part of storage driver's error handler,
> such as nvme driver, when may happen during CPU hotplug, in which
> storage queue has to drain its pending IOs because all CPUs associated
> with the queue are offline and the queue is becoming inactive. And
> handling IO needs error handler to provide forward progress.
>
> Then dead lock is caused:
>
> 1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's
> handler is waiting for inflight IO
>
> 2) error handler is waiting for CPU hotplug lock
>
> 3) inflight IO can't be completed in blk-mq's CPU hotplug handler because
> error handling can't provide forward progress.
>
> Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(),
> in which two stage spreads are taken: 1) the 1st stage is over all present
> CPUs; 2) the end stage is over all other CPUs.
>
> Turns out the two stage spread just needs consistent 'cpu_present_mask', and
> remove the CPU hotplug lock by storing it into one local cache. This way
> doesn't change correctness, because all CPUs are still covered.
>
> Cc: Keith Busch <kbusch@...nel.org>
> Cc: linux-nvme@...ts.infradead.org
> Cc: linux-block@...r.kernel.org
> Reported-by: Yi Zhang <yi.zhang@...hat.com>
> Reported-by: Guangwu Zhang <guazhang@...hat.com>
> Tested-by: Guangwu Zhang <guazhang@...hat.com>
> Reviewed-by: Chengming Zhou <zhouchengming@...edance.com>
> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
> V3:
>         - reuse `npresmsk`, and avoid to allocate new variable, suggested by
>         Chengming Zhou

Hello Thomas and Jens,

Ping...

Thanks,
Ming