Re: question about cpusets vs sched_setaffinity()

[Jason Baron <jbaron@akamai.com>]

On 12/10/2015 04:30 PM, Chris Friesen wrote:
> Hi,
> 
> I've got a question about the interaction between cpusets and
> sched_setaffinity().
> 
> If I put a task into a cpuset and then call sched_setaffinity() on it,
> it will be affined to the intersection of the two sets of cpus.  (Those
> specified on the set, and those specified in the syscall.)
> 
> However, if I then change the cpus in the cpuset the process affinity
> will simply be overwritten by the new cpuset affinity.  It does not seem
> to take into account any restrictions from the original
> sched_setaffinity() call.
> 
> Wouldn't it make more sense to affine the process to the intersection
> between the new set of cpus from the cpuset, and the current process
> affinity?  That way if I explicitly masked out certain CPUs in the
> original sched_setaffinity() call then they would remain masked out
> regardless of changes to the set of cpus assigned to the cpuset.
> 
> Thanks,
> Chris
> 
> PS: Not subscribed to the list, please CC me on replies.

Hi,

This behavior seems a bit odd to me as well - if you've done a
sched_setaffinity() call to a subset of the cpus of a cpuset that the
task in contained within, any change to the cpuset cpus will wipe away
the sched_setaffinity() settings even if they continue to be a subset of
the cpuset cpus.

To add the behavior you are describing, I think requires another
cpumask_t field in the task_struct. Where we could store the last
requested mask value for sched_setaffinity() and use that when updating
the cpus for a cpuset via an intersection as you described. I think
adding a task to a cpuset still should wipe out any sched_setaffinity()
settings - but that would depend on the desired semantics here. It would
also require a knob so as not to break existing behavior by default.

You could also create a child cgroup for the process that you don't want
to change and set the cpus on that cgroup instead of using
sched_setaffinity(). Then you change the cpus for the parent cgroup and
that shouldn't affect the child as long as the child cgroup is a subset.
But its not entirely clear to me if that addresses your use-case?

Thanks,

-Jason