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@xxxxxxxxxx>
> > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Reviewed-by: Quentin Perret <qperret@xxxxxxxxxx>
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > ---
> > 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.
