linux-kernel - Re: [PATCH v4 1/2] sched/rt: fix bad task migration for rt tasks

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Message-ID: <20220708172614.14191089@gandalf.local.home>
Date:   Fri, 8 Jul 2022 17:26:14 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Schspa Shi <schspa@...il.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        zhaohui.shi@...izon.ai
Subject: Re: [PATCH v4 1/2] sched/rt: fix bad task migration for rt tasks

On Sat,  9 Jul 2022 05:17:54 +0800
Schspa Shi <schspa@...il.com> wrote:

> +++ b/kernel/sched/rt.c
> @@ -1998,11 +1998,14 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
>  			 * the mean time, task could have
>  			 * migrated already or had its affinity changed.
>  			 * Also make sure that it wasn't scheduled on its rq.
> +			 * It is possible the task has running for a while,

I don't understand the "running for a while" part. That doesn't make sense.

The only way this can happen is that it was scheduled, set
"migrate_disabled" and then got preempted where it's no longer on the run
queue.

-- Steve


> +			 * And we check task migration disable flag here.
>  			 */
>  			if (unlikely(task_rq(task) != rq ||
>  				     !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_mask) ||
>  				     task_running(rq, task) ||
>  				     !rt_task(task) ||
> +				     is_migration_disabled(task) ||
>  				     !task_on_rq_queued(task))) {
>  
>  				double_unlock_balance(rq, lowest_rq);