Re: [PATCH] sched/fair: Fix wrong cpu selecting from isolated domain

[Jiang Biao]

On Tue, 25 Aug 2020 at 17:28, xunlei <xlpang@xxxxxxxxxxxxxxxxx> wrote:
>
> On 2020/8/25 下午2:37, Jiang Biao wrote:
> > On Mon, 24 Aug 2020 at 20:31, Xunlei Pang <xlpang@xxxxxxxxxxxxxxxxx> wrote:
> >>
> >> We've met problems that occasionally tasks with full cpumask
> >> (e.g. by putting it into a cpuset or setting to full affinity)
> >> were migrated to our isolated cpus in production environment.
> >>
> >> After some analysis, we found that it is due to the current
> >> select_idle_smt() not considering the sched_domain mask.
> >>
> >> Fix it by checking the valid domain mask in select_idle_smt().
> >>
> >> Fixes: 10e2f1acd010 ("sched/core: Rewrite and improve select_idle_siblings())
> >> Reported-by: Wetp Zhang <wetp.zy@xxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxxxxxxxxx>
> >> ---
> >>  kernel/sched/fair.c | 9 +++++----
> >>  1 file changed, 5 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 1a68a05..fa942c4 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -6075,7 +6075,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
> >>  /*
> >>   * Scan the local SMT mask for idle CPUs.
> >>   */
> >> -static int select_idle_smt(struct task_struct *p, int target)
> >> +static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
> >>  {
> >>         int cpu;
> >>
> >> @@ -6083,7 +6083,8 @@ static int select_idle_smt(struct task_struct *p, int target)
> >>                 return -1;
> >>
> >>         for_each_cpu(cpu, cpu_smt_mask(target)) {
> >> -               if (!cpumask_test_cpu(cpu, p->cpus_ptr))
> >> +               if (!cpumask_test_cpu(cpu, p->cpus_ptr) ||
> >> +                   !cpumask_test_cpu(cpu, sched_domain_span(sd)))
> > Maybe the following change could be better, :)
> > for_each_cpu_and(cpu, cpu_smt_mask(target), sched_domain_span(sd))
> > keep a similar style with select_idle_core/cpu, and could reduce loops.
> >
>
> I thought that, but given that smt mask is usually small, the original
> code may run a bit faster?
Not sure. :)
It's OK for me.

Regards,
Jiang