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SubjectRe: [PATCH v3 6/6] sched/rt: Fix pushing unfit tasks to a better CPU
Hi Pavan

On 03/02/20 13:27, Qais Yousef wrote:
> If a task was running on unfit CPU we could ignore migrating if the
> priority level of the new fitting CPU is the *same* as the unfit one.
>
> Add an extra check to select_task_rq_rt() to allow the push in case:
>
> * p->prio == new_cpu.highest_priority
> * task_fits(p, new_cpu)
>
> Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> ---

Can you please confirm if you have any objection to this patch? Without it
I see large delays in the 2 tasks test like I outlined in [1]. It wasn't clear
from [2] whether you are in agreement now or not.

[1] https://lore.kernel.org/lkml/20200217135306.cjc2225wdlwqiicu@e107158-lin.cambridge.arm.com/
[2] https://lore.kernel.org/lkml/20200227033608.GN28029@codeaurora.org/

Thanks

--
Qais Yousef

> kernel/sched/rt.c | 41 ++++++++++++++++++++++++++++-------------
> 1 file changed, 28 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ce230bec6847..8aaa442e4867 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1474,20 +1474,35 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
> if (test || !rt_task_fits_capacity(p, cpu)) {
> int target = find_lowest_rq(p);
>
> - /*
> - * Bail out if we were forcing a migration to find a better
> - * fitting CPU but our search failed.
> - */
> - if (!test && target != -1 && !rt_task_fits_capacity(p, target))
> - goto out_unlock;
> + if (target != -1) {
> + bool fit_target = rt_task_fits_capacity(p, target);
>
> - /*
> - * Don't bother moving it if the destination CPU is
> - * not running a lower priority task.
> - */
> - if (target != -1 &&
> - p->prio < cpu_rq(target)->rt.highest_prio.curr)
> - cpu = target;
> + /*
> + * Bail out if we were forcing a migration to find a
> + * better fitting CPU but our search failed.
> + */
> + if (!test && !fit_target)
> + goto out_unlock;
> +
> + /*
> + * Don't bother moving it if the destination CPU is
> + * not running a lower priority task.
> + */
> + if (p->prio < cpu_rq(target)->rt.highest_prio.curr) {
> +
> + cpu = target;
> +
> + } else if (p->prio == cpu_rq(target)->rt.highest_prio.curr) {
> +
> + /*
> + * If the priority is the same and the new CPU
> + * is a better fit, then move, otherwise don't
> + * bother here either.
> + */
> + if (fit_target)
> + cpu = target;
> + }
> + }
> }
>
> out_unlock:
> --
> 2.17.1
>

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