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 <[email protected]> > Reported-by: Mark Simmons <[email protected]> > Reviewed-by: Juri Lelli <[email protected]> > Tested-by: Mark Simmons <[email protected]> > Cc: Ingo Molnar <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Juri Lelli <[email protected]> > Cc: Vincent Guittot <[email protected]> > Cc: Dietmar Eggemann <[email protected]> > Cc: Steven Rostedt <[email protected]> > Cc: Ben Segall <[email protected]> > Cc: Mel Gorman <[email protected]> > Cc: Daniel Bristot de Oliveira <[email protected]> > Cc: [email protected] > > ---
Thanks for this fix. Acked-by: Juri Lelli <[email protected]> Best, Juri
