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

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Message-ID: <87zg274m8u.ffs@tglx>
Date:   Thu, 31 Aug 2023 11:03:29 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, 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

Ming!

On Thu, Aug 31 2023 at 16:15, Ming Lei wrote:
> On Wed, Aug 30, 2023 at 07:44:03PM +0200, Thomas Gleixner wrote:
> There isn't difference compared with holding cpu hotplug lock, because
> the cpu hp state is always changed even though cpu_present_mask isn't
> updated with the lock.

Updates to cpu_present_mask happen with the cpus lock write locked:

acpi_processor_hotadd_init()
  cpu_maps_update_begin()
  cpus_write_lock()
  acpi_map_cpu()
    generic_processor_info()
      set_cpu_present(cpu, true);

The only CPU mask which is lockless race free accessible is
cpus_possible_mask because that one is set up at boot once and never
modified afterwards.

> And all CPUs are always covered during the spread, the difference
> is just that it is done in 1st stage or 2nd stage.

I'm not objecting against the approach per se. It just needs proper
documentation why this is correct.

Thanks,

        tglx