linux-kernel - Re: [PATCH v15 7/7] timers: Exclude isolated cpus from timer migration

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Message-ID: <87jyzllwhd.ffs@tglx>
Date: Wed, 19 Nov 2025 19:15:42 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Gabriele Monaco <gmonaco@...hat.com>, linux-kernel@...r.kernel.org,
 Anna-Maria Behnsen <anna-maria@...utronix.de>, Waiman Long
 <longman@...hat.com>, "John B. Wyatt IV" <jwyatt@...hat.com>, "John B.
 Wyatt IV" <sageofredondo@...il.com>
Subject: Re: [PATCH v15 7/7] timers: Exclude isolated cpus from timer migration

On Wed, Nov 19 2025 at 18:14, Frederic Weisbecker wrote:
> Le Wed, Nov 19, 2025 at 05:50:15PM +0100, Thomas Gleixner a écrit :
>> But thinking more about it. What's the actual point of moving this 'clear'
>> out instead of just moving it further down?
>> 
>> It does not matter at all whether the isol/unisol muck clears an already
>> cleared bit or not. But it would keep the function name comprehensible
>> and avoid all this online/offline wrapper nonsense.
>
> That was my suggestion.
>
> It's because tmigr_clear_cpu_available() and tmigr_set_cpu_available()
> can now all be called concurrently through the workqueues and race and
> mess up the cpumask if they all try to clear/set at the same time...

Huch?

cpumask_set_cpu() uses set_bit() and cpumask_clear_cpu() uses
clear_bit(). Both are atomic and nothing gets messed up.

The only undefined case would be if you end up setting/clearing the same
bit, which would require that the unisol and isol maps overlap. But that
would be a bug on it's own, no?

Thanks,

        tglx