linux-kernel - Re: [PATCH v2] cpu/hotplug: wait for cpuset_hotplug

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Message-ID: <20210216182925.v2fe3foe6dgmiaj6@e107158-lin.cambridge.arm.com>
Date:   Tue, 16 Feb 2021 18:29:25 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Alexey Klimov <aklimov@...hat.com>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        peterz@...radead.org, yury.norov@...il.com,
        daniel.m.jordan@...cle.com, tglx@...utronix.de, jobaker@...hat.com,
        audralmitchel@...il.com, arnd@...db.de, gregkh@...uxfoundation.org,
        rafael@...nel.org, tj@...nel.org, hannes@...xchg.org,
        klimov.linux@...il.com
Subject: Re: [PATCH v2] cpu/hotplug: wait for cpuset_hotplug_work to finish
 on cpu onlining

On 02/12/21 00:30, Alexey Klimov wrote:
> When a CPU offlined and onlined via device_offline() and device_online()
> the userspace gets uevent notification. If, after receiving "online" uevent,
> userspace executes sched_setaffinity() on some task trying to move it
> to a recently onlined CPU, then it often fails with -EINVAL. Userspace needs
> to wait around 5..30 ms before sched_setaffinity() will succeed for recently
> onlined CPU after receiving uevent.
> 
> If in_mask argument for sched_setaffinity() has only recently onlined CPU,
> it often fails with such flow:
> 
>   sched_setaffinity()
>     cpuset_cpus_allowed()
>       guarantee_online_cpus()   <-- cs->effective_cpus mask does not
>                                         contain recently onlined cpu
>     cpumask_and()               <-- final new_mask is empty
>     __set_cpus_allowed_ptr()
>       cpumask_any_and_distribute() <-- returns dest_cpu equal to nr_cpu_ids
>       returns -EINVAL
> 
> Cpusets used in guarantee_online_cpus() are updated using workqueue from
> cpuset_update_active_cpus() which in its turn is called from cpu hotplug callback
> sched_cpu_activate() hence it may not be observable by sched_setaffinity() if
> it is called immediately after uevent.

nit: newline

> Out of line uevent can be avoided if we will ensure that cpuset_hotplug_work
> has run to completion using cpuset_wait_for_hotplug() after onlining the
> cpu in cpu_device_up() and in cpuhp_smt_enable().
> 
> Co-analyzed-by: Joshua Baker <jobaker@...hat.com>
> Signed-off-by: Alexey Klimov <aklimov@...hat.com>
> ---

This looks good to me.

Reviewed-by: Qais Yousef <qais.yousef@....com>

Thanks

--
Qais Yousef