From: Daniel Bristot de Oliveira bristot@redhat.com
commit feff2e65efd8d84cf831668e182b2ce73c604bbb upstream.
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.
Reported-by: Mark Simmons msimmons@redhat.com Signed-off-by: Daniel Bristot de Oliveira bristot@redhat.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Juri Lelli juri.lelli@redhat.com Tested-by: Mark Simmons msimmons@redhat.com Link: https://lkml.kernel.org/r/5076e003450835ec74e6fa5917d02c4fa41687e6.160017029... [Ankit: Regenerated the patch for v5.4.y] Signed-off-by: Ankit Jain ankitja@vmware.com --- kernel/sched/deadline.c | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+)
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 2bda9fdba31c..fdeb2afffc48 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -1484,6 +1484,27 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags) */ if (pi_task && dl_prio(pi_task->normal_prio) && p->dl.dl_boosted) { pi_se = &pi_task->dl; + /* + * Because of delays in the detection of the overrun of a + * thread's runtime, it might be the case that a thread + * goes to sleep in a rt mutex with negative runtime. As + * a consequence, the thread will be throttled. + * + * While waiting for the mutex, this thread can also be + * boosted via PI, resulting in a thread that is throttled + * and boosted at the same time. + * + * In this case, the boost overrides the throttle. + */ + if (p->dl.dl_throttled) { + /* + * The replenish timer needs to be canceled. No + * problem if it fires concurrently: boosted threads + * are ignored in dl_task_timer(). + */ + hrtimer_try_to_cancel(&p->dl.dl_timer); + p->dl.dl_throttled = 0; + } } else if (!dl_prio(p->normal_prio)) { /* * Special case in which we have a !SCHED_DEADLINE task