Hi, On 13/03/19 15:49, luca abeni wrote: > Hi, > > (I added Juri in cc) > > On Tue, 12 Mar 2019 10:03:12 +0800 > "chengjian (D)" <[email protected]> wrote: > [...] > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > > index 31c050a0d0ce..d73cb033a06d 100644 > > --- a/kernel/sched/deadline.c > > +++ b/kernel/sched/deadline.c > > @@ -252,7 +252,6 @@ static void task_non_contending(struct > > task_struct *p) if (dl_entity_is_special(dl_se)) > > return; > > > > - WARN_ON(hrtimer_active(&dl_se->inactive_timer)); > > WARN_ON(dl_se->dl_non_contending); > > > > zerolag_time = dl_se->deadline - > > @@ -287,7 +286,9 @@ static void task_non_contending(struct > > task_struct *p) } > > > > dl_se->dl_non_contending = 1; > > - get_task_struct(p); > > + > > + if (!hrtimer_active(&dl_se->inactive_timer)); > > + get_task_struct(p); > > hrtimer_start(timer, ns_to_ktime(zerolag_time), > > HRTIMER_MODE_REL); } > > After looking at the patch a little bit more and running some tests, > I suspect this solution might be racy: > when the timer is already active, (and hrtimer_start() fails), it > relies on its handler to decrease the running bw (by setting > dl_non_contending to 1)... But inactive_task_timer() might have > already checked dl_non_contending, finding it equal to 0 (so, it > ends up doing nothing and the running bw is not decreased). > > > So, I would prefer a different solution. I think this patch should work: > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index 6a73e41a2016..43901fa3f269 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -252,7 +252,6 @@ static void task_non_contending(struct task_struct *p) > if (dl_entity_is_special(dl_se)) > return; > > - WARN_ON(hrtimer_active(&dl_se->inactive_timer)); > WARN_ON(dl_se->dl_non_contending); > > zerolag_time = dl_se->deadline - > @@ -269,7 +268,7 @@ static void task_non_contending(struct task_struct *p) > * If the "0-lag time" already passed, decrease the active > * utilization now, instead of starting a timer > */ > - if (zerolag_time < 0) { > + if ((zerolag_time < 0) || hrtimer_active(&dl_se->inactive_timer)) { > if (dl_task(p)) > sub_running_bw(dl_se, dl_rq); > if (!dl_task(p) || p->state == TASK_DEAD) { > > > The idea is that if the timer is active, we leave dl_non_contending set to > 0 (so that the timer handler does nothing), and we immediately decrease the > running bw. > I think this is OK, because this situation can happen only if the task > blocks, wakes up while the timer handler is running, and then immediately > blocks again - while the timer handler is still running. So, the "zero lag > time" cannot be too much in the future.
And if we get here and the handler is running it means that the handler is spinning on rq->lock waiting the dequeue to release it. So, this looks safe to me as well. BTW, I could reproduce with Steve's deadline_test [1], and this seems to fix it. Would you mind sending out a proper patch Luca? Thanks! - Juri 1 - https://goo.gl/fVbRSu
