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 <bris...@redhat.com> >> Reported-by: Mark Simmons <msimm...@redhat.com> >> Reviewed-by: Juri Lelli <juri.le...@redhat.com> >> Tested-by: Mark Simmons <msimm...@redhat.com> >> Cc: Ingo Molnar <mi...@redhat.com> >> Cc: Peter Zijlstra <pet...@infradead.org> >> Cc: Juri Lelli <juri.le...@redhat.com> >> Cc: Vincent Guittot <vincent.guit...@linaro.org> >> Cc: Dietmar Eggemann <dietmar.eggem...@arm.com> >> Cc: Steven Rostedt <rost...@goodmis.org> >> Cc: Ben Segall <bseg...@google.com> >> Cc: Mel Gorman <mgor...@suse.de> >> Cc: Daniel Bristot de Oliveira <bris...@redhat.com> >> Cc: linux-kernel@vger.kernel.org >> >> --- > > Thanks for this fix. > > Acked-by: Juri Lelli <juri.le...@redhat.com>
This is a gentle ping... [we are facing this bug in practice :-(]. -- Daniel > Best, > Juri >
