linux-kernel - Re: [PATCH v7 1/2] sched/rt: fix bad task migration for rt tasks

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Date:   Sun, 28 Aug 2022 23:45:38 +0800
From:   Schspa Shi <schspa@...il.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/2] sched/rt: fix bad task migration for rt tasks


Dietmar Eggemann <dietmar.eggemann@....com> writes:

> On 13/07/2022 15:48, Schspa Shi wrote:
>> Commit 95158a89dd50 ("sched,rt: Use the full cpumask for balancing")
>> allow find_lock_lowest_rq to pick a task with migration disabled.
>> This commit is intended to push the current running task on this CPU
>> away.
>> 
>> There is a race scenario, which allows a migration disabled task to
>> be migrated to another CPU.
>> 
>> When there is a RT task with higher priority, rt sched class was
>> intended to migrate higher priority task to lowest rq via push_rt_tasks,
>> this BUG will happen here.
>        ^^^
>
> You mean the warning in set_task_cpu()?
>

Yes, this is the warning in set_task_cpu()?

void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
{
#ifdef CONFIG_SCHED_DEBUG
	...
	WARN_ON_ONCE(is_migration_disabled(p));
#endif
	...
}

>> With the system running on PREEMPT_RT, rt_spin_lock will disable
>> migration, this will make the problem easier to reproduce.
>> 
>> I have seen this crash on PREEMPT_RT, from the logs, there is a race
>                    ^^^^^
>                      ?
>
> We still talking about the set_task_cpu() warning, right?
>

Yes.

> [...]
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
>
> You should carry the Reviewed-by:'s you got in previous versions.

Thanks for the reminder, I'll pay attention to this next time.

I will upload a new patch version to carry it. and change the BUG in
comment message to WARN.

-- 
BRs
Schspa Shi