Re: [PATCH] sched: Improve rq selection for a blocked task when its affinity changes

[Ze Gao]

On Thu, Mar 14, 2024 at 10:59 AM Ze Gao <zegao2021@xxxxxxxxx> wrote:
>
> On Wed, Mar 13, 2024 at 11:16 PM Pierre Gondois <pierre.gondois@xxxxxxx> wrote:
> >
> > Hello Ze,
> >
> > I am running stress-ng with the following command:
> >    stress-ng -c 1 -l 10 &
> > and migrating the process with:
> >    taskset -pc [cpus] [pid]
> >
> > The thread seems to be migrated via:
> > sched_setaffinity
> >    \-__sched_setaffinity()
> >      \-__set_cpus_allowed_ptr()
> >        \-__set_cpus_allowed_ptr_locked()
> >          \- [1]
>
>
>
> > [1]
> > /*
> >   * Picking a ~random cpu helps in cases where we are changing affinity
> >   * for groups of tasks (ie. cpuset), so that load balancing is not
> >   * immediately required to distribute the tasks within their new mask.
> >   */
> > dest_cpu = cpumask_any_and_distribute(cpu_valid_mask, ctx->new_mask);
> >
> > So it seems the destination CPU chosen among the new CPU affinity mask is done
> > here, by picking a random CPU in the mask.
>
> IIUC, this is for running/queued/waking tasks instead of blocked tasks.
>
> Am I missing something obvious here?
>
> > Checking the cpus_ptr in select_idle_sibling() might be useful in other cases,
> > but I think the experiment doesn't show that. Maybe a another small tweak could
>
> The experiment is used to illustrate that the status quo does not do well
> but has to rely on select_fallback_rq() to choose a cpu for a woken task
> which turns out to be a bad choice since it's already monopolized by a
> cpu bound task, that is why a second migration happens with the help
> of the load balancer.

Btw, perf alone does not show obvious results here, you need some
other observability tools to make sure the migration is not initiated by
__set_cpus_allowed_ptr_locked (i.e., for running tasks). I achieve this
by directly adding some tracepoints to both select_fallback_rq() and
select_idle_sibling().

Regards,
        -- Ze

> Actually, we can reuse the same reasons for doing so as in
>
>     commit 46a87b3851f0("sched/core: Distribute tasks within affinity masks")
>
> > be done at [1] instead ?
>
> As for blocked tasks, check out what is commented on set_task_cpu() and
> select_task_rq(), since we never call set_task_cpu() on blocked tasks which
> in turn, we have no way to change p->wake_cpu to dest_cpu being randomly
> chosen here, so when it's woken up, it still needs to go through the
> select_task_rq() process using the outdated p->wake_cpu.
>
>
> Thanks,
>         -- Ze
>
> > Regards,
> > Pierre
> >
> > On 3/13/24 09:58, Ze Gao wrote:
> > > We observered select_idle_sibling() is likely to return the *target* cpu
> > > early which is likely to be the previous cpu this task is running on even
> > > when it's actually not within the affinity list newly set, from where after
> > > we can only rely on select_fallback_rq() to choose one for us at its will
> > > (the first valid mostly for now).
> > >
> > > However, the one chosen by select_fallback_rq() is highly likely not a
> > > good enough candidate, sometimes it has to rely on load balancer to kick
> > > in to place itself to a better cpu, which adds one or more unnecessary
> > > migrations in no doubt. For example, this is what I get when I move task
> > > 3964 to cpu 23-24 where cpu 23 has a cpu bound work pinned already:
> > >
> > >          swapper       0 [013]   959.791829: sched:sched_migrate_task: comm=stress-ng-cpu pid=3964 prio=120 orig_cpu=13 dest_cpu=23
> > > kworker/24:2-mm    1014 [024]   959.806148: sched:sched_migrate_task: comm=stress-ng-cpu pid=3964 prio=120 orig_cpu=23 dest_cpu=24
> > >
> > > The thing is we can actually do better if we do checks early and take more
> > > advantages of the *target* in select_idle_sibling(). That is, we continue
> > > the idle cpu selection if *target* fails the test of cpumask_test_cpu(
> > > *target*, p->cpus_ptr). By doing so, we are likely to pick a good candidate,
> > > especially when the newly allowed cpu set shares some cpu resources with
> > > *target*.
> > >
> > > And with this change, we clearly see the improvement when I move task 3964
> > > to cpu 25-26 where cpu 25 has a cpu bound work pinned already.
> > >
> > >          swapper       0 [027]  4249.204658: sched:sched_migrate_task: comm=stress-ng-cpu pid=3964 prio=120 orig_cpu=27 dest_cpu=26
> > >
> > > Note we do the same check for *prev* in select_idle_sibling() as well.
> > >
> > > Signed-off-by: Ze Gao <zegao@xxxxxxxxxxx>
> > > ---
> > >   kernel/sched/fair.c | 13 ++++++++-----
> > >   1 file changed, 8 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 533547e3c90a..9ef6e74c6b2a 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -7511,16 +7511,19 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> > >        */
> > >       lockdep_assert_irqs_disabled();
> > >
> > > -     if ((available_idle_cpu(target) || sched_idle_cpu(target)) &&
> > > -         asym_fits_cpu(task_util, util_min, util_max, target))
> > > +     if (cpumask_test_cpu(target, p->cpus_ptr) &&
> > > +             (available_idle_cpu(target) || sched_idle_cpu(target)) &&
> > > +             asym_fits_cpu(task_util, util_min, util_max, target))
> > >               return target;
> > >
> > >       /*
> > >        * If the previous CPU is cache affine and idle, don't be stupid:
> > >        */
> > > -     if (prev != target && cpus_share_cache(prev, target) &&
> > > -         (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
> > > -         asym_fits_cpu(task_util, util_min, util_max, prev)) {
> > > +     if (cpumask_test_cpu(prev, p->cpus_ptr) &&
> > > +             prev != target &&
> > > +             cpus_share_cache(prev, target) &&
> > > +             (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
> > > +             asym_fits_cpu(task_util, util_min, util_max, prev)) {
> > >
> > >               if (!static_branch_unlikely(&sched_cluster_active) ||
> > >                   cpus_share_resources(prev, target))