[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <fe46e855-b812-c1c5-07ce-7a3715bcbd2e@redhat.com>
Date: Thu, 3 Aug 2023 13:43:34 -0400
From: Waiman Long <longman@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: 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 v2] sched/core: Use empty mask to reset cpumasks in
sched_setaffinity()
On 7/21/23 05:42, Peter Zijlstra wrote:
> On Mon, Jul 17, 2023 at 02:02:43PM -0400, Waiman Long wrote:
>> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
>> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
>> perserved even if the task is being moved to a different cpuset. However,
>> that affinity is also being inherited by any subsequently created child
>> processes which may not want or be aware of that affinity.
>>
>> One way to solve this problem is to provide a way to back off from
>> that user provided CPU affinity. This patch implements such a scheme
>> by using an empty cpumask to signal a reset of the cpumasks to the
>> default as allowed by the current cpuset.
>>
>> Before this patch, passing in an empty cpumask to sched_setaffinity(2)
>> will always return an EINVAL error. With this patch, an error will no
>> longer be returned if sched_setaffinity(2) has been called before to
>> set up user_cpus_ptr. Instead, the user_cpus_ptr that stores the user
>> provided affinity will be cleared and the task's CPU affinity will be
>> reset to that of the current cpuset. No error will be returned in this
>> case to signal that a reset has happened.
>>
>> If sched_setaffinity(2) has not been called previously, an EINVAL error
>> will be returned with an empty cpumask just like before. As a result,
>> tests or tools that rely on this behavior will not be affected unless
>> they have somehow called sched_setaffinity(2) before.
>>
>> We will have to update the sched_setaffinity(2) manpage to document
>> this possible side effect of passing in an empty cpumask.
> So a normal task, that hasn't had it's affinity changed will have
> possible_mask.
>
> So why not use in_mask == possible_mask to clear the user state?
It is not straight forward for a user application to figure what exactly
is possible_mask. Using a empty mask, however, is much easier with the
CPU_ZERO() macro.
In many cases, tasks may be running under a cpuset with cpu list that is
a proper subset of possible_mask. Since possible_mask is a valid
cpumask, we can't use it to reset to the cpuset's default which is what
this patch is.
Cheers,
Longman
Powered by blists - more mailing lists