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SubjectRe: [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@redhat.com>
> >> Reported-by: Mark Simmons <msimmons@redhat.com>
> >> Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
> >> Tested-by: Mark Simmons <msimmons@redhat.com>
> >> Cc: Ingo Molnar <mingo@redhat.com>
> >> Cc: Peter Zijlstra <peterz@infradead.org>
> >> Cc: Juri Lelli <juri.lelli@redhat.com>
> >> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> >> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> >> Cc: Steven Rostedt <rostedt@goodmis.org>
> >> Cc: Ben Segall <bsegall@google.com>
> >> Cc: Mel Gorman <mgorman@suse.de>
> >> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> >> Cc: linux-kernel@vger.kernel.org
> >>
> >> ---
> >
> > Thanks for this fix.
> >
> > Acked-by: Juri Lelli <juri.lelli@redhat.com>
>
> This is a gentle ping... [we are facing this bug in practice :-(].

Sorry, queued now.

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