linux-kernel - Re: [PATCH v7 2/2] sched/rt: Trying to push current task when target disable migrating

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Date:   Fri, 26 Aug 2022 20:46:12 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Schspa Shi <schspa@...il.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com
Cc:     linux-kernel@...r.kernel.org, zhaohui.shi@...izon.ai
Subject: Re: [PATCH v7 2/2] sched/rt: Trying to push current task when target
 disable migrating

On 13/07/2022 15:48, Schspa Shi wrote:
> When the task to push disable migration, retry to push the current
> running task on this CPU away, instead doing nothing for this migrate
> disabled task.
> 
> Signed-off-by: Schspa Shi <schspa@...il.com>

Unfortunately, I can't recreate this issue on my Arm64 6 CPUs system on
mainline or PREEMPT_RT (linux-5.19.y-rt and v5.10.59-rt52) (the one you
mentioned in v6.)

With an rt-app rt workload of 12-18 periodic rt-tasks (4/16ms) all with
different priorities I never ran into a `is_migration_disabled(task)`
situation. I only ever get `task_rq(task) != rq` or `task_running(rq,
task)` under the `if (double_lock_balance(rq, lowest_rq))` condition in
find_lock_lowest_rq().

[...]

>  	// XXX validate p is still the highest prio task
>  	if (task_rq(p) == rq) {
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index cb3b886a081c..21af20445e7f 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2335,6 +2335,15 @@ static int push_dl_task(struct rq *rq)
>  		 */
>  		task = pick_next_pushable_dl_task(rq);
>  		if (task == next_task) {
> +			/*
> +			 * If next task has now disabled migrating, see if we
> +			 * can push the current task.
> +			 */
> +			if (unlikely(is_migration_disabled(task))) {
> +				put_task_struct(next_task);
> +				goto retry;
> +			}
> +

Looks like for DL this makes no sense since we're not pushing rq->curr
in `retry:` like for RT in case `is_migration_disabled(next_task)`.

[...]

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>