Re: question about cpusets vs sched_setaffinity()

[Jason Baron <jbaron@akamai.com>]

On 12/11/2015 06:26 PM, Chris Friesen wrote:
> On 12/11/2015 04:15 PM, Jason Baron wrote:
>> On 12/10/2015 04:30 PM, Chris Friesen wrote:
> 
>>> 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.
> 
> <snip>
> 
>> 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.
> 
> Agreed, the additional field in the task_struct makes sense.  Personally
> I don't think that adding a task to a cpuset should wipe out any
> previously-set affinity, I think it should take the intersection for
> that case as well.
> 
> In this environment it might make sense to have separate queries to
> return the requested and actual affinity.
>

So because cpumask_t is dimensioned by NR_CPUS, I think we would need a
pointer to the cpumask_t field. And we could allocate it when we want
the cpus set by sched_setaffinity() to persist across the cgroup cpuset
cpu changes. I think you are right that a flag to
sched_[set|get]affinity() for this case might be nice - but that would
require a new syscall...

>> 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?
> 
> I ended up doing something like this where I had a top-level cpuset and
> a number of child cpusets, each with an exclusive subset of the CPUs
> assigned to it.  But it meant that I needed more complicated code to
> figure out which tasks needed to go into which child cpusets, and more
> complicated code to handle removing a CPU from the top-level cpuset
> (since you have to remove it from any children first).
> 
> Chris

I agree that it would be nice to improve this interface, since you are
creating extra cgroups here just to sort of work around this.

Thanks,

-Jason