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

[Pierre Gondois]

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.

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
be done at [1] instead ?

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))