linux-kernel - Re: [PATCH v4] sched/core: Use zero length to reset cpumasks in sched

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Message-ID: <0c36c117-b44c-89ef-1988-6059c41fad65@redhat.com>
Date:   Wed, 4 Oct 2023 12:23:28 -0400
From:   Waiman Long <longman@...hat.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org, Phil Auld <pauld@...hat.com>,
        Brent Rowsell <browsell@...hat.com>,
        Peter Hunt <pehunt@...hat.com>
Subject: Re: [PATCH v4] sched/core: Use zero length to reset cpumasks in
 sched_setaffinity()


On 10/4/23 08:55, Florian Weimer wrote:
> * Waiman Long:
>
>> On 10/4/23 08:34, Florian Weimer wrote:
>>> * Peter Zijlstra:
>>>
>>>> Subject: sched: Add CPU_FILL()
>>>>
>>>> Add the CPU_FILL() macros to easily create an all-set cpumask.
>>>>
>>>> FreeBSD also provides this macro with this semantic.
>>>>
>>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>>> My main concer is that obtaining the size of the mask, or at least an
>>> approximiation is not exactly easy.  If there's an expectation that
>>> applications reset the mask more often than they do today (I don't have
>>> the full context here), then we'd some decent interface to get the
>>> approriate size.
>> I believe the macro just use sizeof(cpu_set_t) as the size of the
>> bitmask. It is the same case as in CPU_ZERO().
> I mean the CPU_FILL_S macro also defined in the patch.  Correctly
> written applications should not use CPU_FILL and statically sized CPU
> sets.

Right, that can be a problem. If the input bitmask size is less than 
cpumask_size(), CPU_FILL_S() won't work to reset the cpumask. In fact, 
it will treat that bitmask just like a regular sched_setaffinity() call 
and set it accordingly.

With that caveat, I would prefer to keep using a length of 0 for the 
reset then.

Cheers,
Longman