linux-kernel - Re: [PATCH] sched/deadline: Unthrottle PI boosted threads while enqueuing

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Date:   Fri, 2 Oct 2020 18:01:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     Juri Lelli <juri.lelli@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Mark Simmons <msimmons@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/deadline: Unthrottle PI boosted threads while
 enqueuing

On Fri, Oct 02, 2020 at 05:57:52PM +0200, Daniel Bristot de Oliveira wrote:
> On 9/18/20 8:00 AM, Juri Lelli wrote:
> > Hi Daniel,
> > 
> > On 16/09/20 09:06, Daniel Bristot de Oliveira wrote:
> >> stress-ng has a test (stress-ng --cyclic) that creates a set of threads
> >> under SCHED_DEADLINE with the following parameters:
> >>
> >>     dl_runtime   =  10000 (10 us)
> >>     dl_deadline  = 100000 (100 us)
> >>     dl_period    = 100000 (100 us)
> >>
> >> These parameters are very aggressive. When using a system without HRTICK
> >> set, these threads can easily execute longer than the dl_runtime because
> >> the throttling happens with 1/HZ resolution.
> >>
> >> During the main part of the test, the system works just fine because
> >> the workload does not try to run over the 10 us. The problem happens at
> >> the end of the test, on the exit() path. During exit(), the threads need
> >> to do some cleanups that require real-time mutex locks, mainly those
> >> related to memory management, resulting in this scenario:
> >>
> >> Note: locks are rt_mutexes...
> >>  ------------------------------------------------------------------------
> >>     TASK A:		TASK B:				TASK C:
> >>     activation
> >> 							activation
> >> 			activation
> >>
> >>     lock(a): OK!	lock(b): OK!
> >>     			<overrun runtime>
> >>     			lock(a)
> >>     			-> block (task A owns it)
> >> 			  -> self notice/set throttled
> >>  +--<			  -> arm replenished timer
> >>  |    			switch-out
> >>  |    							lock(b)
> >>  |    							-> <C prio > B prio>
> >>  |    							-> boost TASK B
> >>  |  unlock(a)						switch-out
> >>  |  -> handle lock a to B
> >>  |    -> wakeup(B)
> >>  |      -> B is throttled:
> >>  |        -> do not enqueue
> >>  |     switch-out
> >>  |
> >>  |
> >>  +---------------------> replenishment timer
> >> 			-> TASK B is boosted:
> >> 			  -> do not enqueue
> >>  ------------------------------------------------------------------------
> >>
> >> BOOM: TASK B is runnable but !enqueued, holding TASK C: the system
> >> crashes with hung task C.
> >>
> >> This problem is avoided by removing the throttle state from the boosted
> >> thread while boosting it (by TASK A in the example above), allowing it to
> >> be queued and run boosted.
> >>
> >> The next replenishment will take care of the runtime overrun, pushing
> >> the deadline further away. See the "while (dl_se->runtime <= 0)" on
> >> replenish_dl_entity() for more information.
> >>
> >> Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
> >> Reported-by: Mark Simmons <msimmons@...hat.com>
> >> Reviewed-by: Juri Lelli <juri.lelli@...hat.com>
> >> Tested-by: Mark Simmons <msimmons@...hat.com>
> >> Cc: Ingo Molnar <mingo@...hat.com>
> >> Cc: Peter Zijlstra <peterz@...radead.org>
> >> Cc: Juri Lelli <juri.lelli@...hat.com>
> >> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> >> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> >> Cc: Steven Rostedt <rostedt@...dmis.org>
> >> Cc: Ben Segall <bsegall@...gle.com>
> >> Cc: Mel Gorman <mgorman@...e.de>
> >> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> >> Cc: linux-kernel@...r.kernel.org
> >>
> >> ---
> > 
> > Thanks for this fix.
> > 
> > Acked-by: Juri Lelli <juri.lelli@...hat.com>
> 
> This is a gentle ping... [we are facing this bug in practice :-(].

Sorry, queued now.