Re: [PATCH] sched/core: Use empty mask to reset cpumasks in sched_setaffinity()

[Waiman Long]

On 7/3/23 06:26, Peter Zijlstra wrote:
> On Wed, Jun 28, 2023 at 05:16:37PM -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 return an EINVAL error. With this patch, an error will no longer
> > be returned. Instead, the user_cpus_ptr that stores the user provided
> > affinity, if set, will be cleared and the task's CPU affinity will be
> > reset to that of the current cpuset. This reverts the cpumask change
> > done by all the previous sched_setaffinity(2) calls.
> This is a user visible ABI change -- but with very limited motivation.
> Why do we want this? Who will use this?

Yes, this is a visible ABI change, but it should be backward compatible 
as I doubt there are applications out there depending on the fact that 
passing an empty cpumask to sched_setaffinity() must return an error.

Our OpenShift team has actually hit a problem with the recent persistent 
user provided cpu affinity change because they are relying on the fact 
that moving a task to a different cpuset will reset cpu affinity to the 
cpuset default which is no longer true. That is the main reason behind 
this patch to provide a way to reset cpu affinity to the cpuset default.

I am thinking of requesting sched_setaffinity(2) manpage update to 
document the persistent user provided cpu affinity change and the way to 
reset it after this patch is merged upstream.

Cheers,
Longman