linux-kernel - Re: [RT] BUG in sched/cpupri.c

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Message-ID: <88826618-5ce8-dd1f-c9db-ec273fede3ce@arm.com>
Date:   Fri, 7 Jan 2022 15:25:21 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     John Keeping <john@...anate.com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-rt-users@...r.kernel.org, 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>,
        linux-kernel@...r.kernel.org
Subject: Re: [RT] BUG in sched/cpupri.c

On 07/01/2022 12:49, John Keeping wrote:
> On Fri, Jan 07, 2022 at 11:46:45AM +0100, Dietmar Eggemann wrote:
>> On 22/12/2021 20:48, Valentin Schneider wrote:
>>>  /*
>>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>>> index ef8228d19382..8f3e3a1367b6 100644
>>> --- a/kernel/sched/rt.c
>>> +++ b/kernel/sched/rt.c
>>> @@ -1890,6 +1890,16 @@ static int push_rt_task(struct rq *rq, bool pull)
>>>  	if (!next_task)
>>>  		return 0;
>>>  
>>> +	/*
>>> +	 * It's possible that the next_task slipped in of higher priority than
>>> +	 * current, or current has *just* changed priority.  If that's the case
>>> +	 * just reschedule current.
>>> +	 */
>>> +	if (unlikely(next_task->prio < rq->curr->prio)) {
>>> +		resched_curr(rq);
>>> +		return 0;
>>> +	}
>>
>> IMHO, that's the bit which prevents the BUG.
>>
>> But this would also prevent the case in which rq->curr is an RT task
>> with lower prio than next_task.
>>
>> Also `rq->curr = migration/X` goes still though which is somehow fine
>> since find_lowest_rq() bails out for if (task->nr_cpus_allowed == 1).
>>
>> And DL tasks (like sugov:X go through and they can have
>> task->nr_cpus_allowed > 1 (arm64 slow-switching boards with shared
>> freuency domains with schedutil). cpupri_find_fitness()->convert_prio()
>> can handle  task_pri, p->prio = -1 (CPUPRI_INVALID) although its somehow
>> by coincidence.
>>
>> So maybe something like this:
> 
> Do you mean to replace just the one hunk from Valentin's patch with the
> change below (keeping the rest), or are you saying that only the change
> below is needed?

The latter.

I think Valentin wanted to see if something like this can also occur via
sched_setscheduler() and maybe for this changes in switched_from_[rt/dl]
will be necessary.

>> @ -1898,6 +1898,11 @@ static int push_rt_task(struct rq *rq, bool pull)
>>                 if (!pull || rq->push_busy)
>>                         return 0;
>>
>> +               if (rq->curr->sched_class != &rt_sched_class) {
>> +                       resched_curr(rq);
>> +                       return 0;
>> +               }
>> +
>>                 cpu = find_lowest_rq(rq->curr);
>>
>> [...