[PATCH v5 07/15] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1

[Peter Zijlstra]

On Thu, Dec 17, 2020 at 12:15:52PM +0000, Qais Yousef wrote:
> On 12/08/20 13:28, Will Deacon wrote:
> > If the scheduler cannot find an allowed CPU for a task,
> > cpuset_cpus_allowed_fallback() will widen the affinity to cpu_possible_mask
> > if cgroup v1 is in use.
> > 
> > In preparation for allowing architectures to provide their own fallback
> > mask, just return early if we're not using cgroup v2 and allow
> > select_fallback_rq() to figure out the mask by itself.
> > 
> > Cc: Li Zefan <lizefan at huawei.com>
> > Cc: Tejun Heo <tj at kernel.org>
> > Cc: Johannes Weiner <hannes at cmpxchg.org>
> > Reviewed-by: Quentin Perret <qperret at google.com>
> > Signed-off-by: Will Deacon <will at kernel.org>
> > ---
> >  kernel/cgroup/cpuset.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index 57b5b5d0a5fd..e970737c3ed2 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -3299,9 +3299,11 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
> >  
> >  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
> >  {
> > +	if (!is_in_v2_mode())
> > +		return; /* select_fallback_rq will try harder */
> > +
> >  	rcu_read_lock();
> > -	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
> > -		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
> > +	do_set_cpus_allowed(tsk, task_cs(tsk)->cpus_allowed);
> 
> Why is it safe to return that for cpuset v2?

v1

Because in that case it does cpu_possible_mask, which, if you look at
select_fallback_rq(), is exactly what happens when cpuset 'fails' to
find a candidate.

Or at least, that's how I read the patch.