linux-kernel - Re: [PATCH] sched/rt: Plug rt_mutex_setprio() vs push_rt

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Date:   Mon, 24 Jan 2022 16:51:10 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org
Cc:     John Keeping <john@...anate.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH] sched/rt: Plug rt_mutex_setprio() vs push_rt_task() race

On 24/01/22 16:47, Dietmar Eggemann wrote:
> On 24/01/2022 14:29, Valentin Schneider wrote:
>> On 24/01/22 10:37, Dietmar Eggemann wrote:
>>> On 20/01/2022 20:40, Valentin Schneider wrote:
>>>
>>> [...]
>>>
>>>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>>>> index 7b4f4fbbb404..48fc8c04b038 100644
>>>> --- a/kernel/sched/rt.c
>>>> +++ b/kernel/sched/rt.c
>>>> @@ -2026,6 +2026,16 @@ static int push_rt_task(struct rq *rq, bool pull)
>>>>  		return 0;
>>>>  
>>>>  retry:
>>>> +	/*
>>>> +	 * It's possible that the next_task slipped in of
>>>> +	 * higher priority than current. If that's the case
>>>> +	 * just reschedule current.
>>>> +	 */
>>>> +	if (unlikely(next_task->prio < rq->curr->prio)) {
>>>> +		resched_curr(rq);
>>>> +		return 0;
>>>> +	}
>>>
>>> If we do this before `is_migration_disabled(next_task), shouldn't then
>>> the related condition in push_dl_task() also be moved up?
>>>
>>>   if (dl_task(rq->curr) &&
>>>     dl_time_before(next_task->dl.deadline, rq->curr->dl.deadline) &&
>>>     rq->curr->nr_cpus_allowed > 1)
>>>
>>> To enforce resched_curr(rq) in the `is_migration_disabled(next_task)`
>>> case there as well?
>>>
>> 
>> I'm not sure if we can hit the same issue with DL since DL doesn't have the
>> push irqwork. If there are DL tasks on the rq when current gets demoted,
>> switched_from_dl() won't queue pull_dl_task().
>
> True. But with your RT change we reschedule current (CFS task or lower
> rt task than next_task) now even in case next task is
> migration-disabled. I.e. we prefer rescheduling over pushing current away.
>
> But for DL we wouldn't reschedule current in such a case, we would just
> return 0.
>
> That said, the prio based check in RT includes other sched classes where
> the DL check only compares DL tasks.
>

I think you got a point to at least align the RT and DL code, and yes we
shouldn't care whether the next pushable DL task is migration_disabled or
not if it's higher prio than current, so I think I'll move that in v2.

>> That said, if say we have DL tasks on the rq and demote the current DL task
>> to RT, do we currently have anything that will call resched_curr() (I'm
>> looking at the rt_mutex path)?
>> switched_to_fair() has a resched_curr() (which helps for the RT -> CFS
>> case), I don't see anything that would give us that in switched_from_dl() /
>> switched_to_rt(), or am I missing something?
>> 
>>>> +
>>>>  	if (is_migration_disabled(next_task)) {
>>>>  		struct task_struct *push_task = NULL;
>>>>  		int cpu;
>>>> @@ -2033,6 +2043,17 @@ static int push_rt_task(struct rq *rq, bool pull)
>>>>  		if (!pull || rq->push_busy)
>>>>  			return 0;
>>>>  
>>>> +		/*
>>>> +		 * Per the above priority check, curr is at least RT. If it's
>>>> +		 * of a higher class than RT, invoking find_lowest_rq() on it
>>>> +		 * doesn't make sense.
>>>> +		 *
>>>> +		 * Note that the stoppers are masqueraded as SCHED_FIFO
>>>> +		 * (cf. sched_set_stop_task()), so we can't rely on rt_task().
>>>> +		 */
>>>> +		if (rq->curr->sched_class != &rt_sched_class)
>>>
>>> s/ != / > / ... since the `unlikely(next_task->prio < rq->curr->prio)`
>>> already filters tasks from lower sched classes (CFS)?
>>>
>> 
>> != points out we won't invoke find_lowest_rq() on anything that isn't RT,
>> which makes it a bit clearer IMO, and it's not like either of those
>> comparisons is more expensive than the other :)
>
> Also true, but it would be more aligned to the comment above '...  If it
> (i.e. curr) 's of a higher class than ...'
>

Right, I can clean that up!

> [...]