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

[Ze Gao]

On Wed, Mar 13, 2024 at 11:16 PM Pierre Gondois <pierre.gondois@armcom> 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.

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select_task_rq() process using the outdated p-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))