On Tue, Apr 09, 2019 at 02:38:55PM -0400, Julien Desfossez wrote: > We found the source of the major performance regression we discussed > previously. It turns out there was a pattern where a task (a kworker in this > case) could be woken up, but the core could still end up idle before that > task had a chance to run. > > Example sequence, cpu0 and cpu1 and siblings on the same core, task1 and > task2 are in the same cgroup with the tag enabled (each following line > happens in the increasing order of time): > - task1 running on cpu0, task2 running on cpu1 > - sched_waking(kworker/0, target_cpu=cpu0) > - task1 scheduled out of cpu0 > - kworker/0 cannot run on cpu0 because of task2 is still running on cpu1 > cpu0 is idle > - task2 scheduled out of cpu1
But at this point core_cookie is still set; we don't clear it when the last task goes away. > - cpu1 doesn’t select kworker/0 for cpu0, because the optimization path ends > the task selection if core_cookie is NULL for currently selected process > and the cpu1’s runqueue. But at this point core_cookie is still set, we only (re)set it later to p->core_cookie. What I suspect happens is that you hit the 'again' clause due to a higher prio @max on the second sibling. And at that point we've destroyed core_cookie. > - cpu1 is idle > --> both siblings are idle but kworker/0 is still in the run queue of cpu0. > Cpu0 may stay idle for longer if it goes deep idle. > > With the fix below, we ensure to send an IPI to the sibling if it is idle > and has tasks waiting in its runqueue. > This fixes the performance issue we were seeing. > > Now here is what we can measure with a disk write-intensive benchmark: > - no performance impact with enabling core scheduling without any tagged > task, > - 5% overhead if one tagged task is competing with an untagged task, > - 10% overhead if 2 tasks tagged with a different tag are competing > against each other. > > We are starting more scaling tests, but this is very encouraging ! > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index e1fa10561279..02c862a5e973 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3779,7 +3779,22 @@ pick_next_task(struct rq *rq, struct task_struct > *prev, struct rq_flags *rf) > > trace_printk("unconstrained pick: %s/%d %lx\n", > next->comm, next->pid, > next->core_cookie); > + rq->core_pick = NULL; > > + /* > + * If the sibling is idling, we might want to > wake it > + * so that it can check for any runnable but > blocked tasks > + * due to previous task matching. > + */ > + for_each_cpu(j, smt_mask) { > + struct rq *rq_j = cpu_rq(j); > + rq_j->core_pick = NULL; > + if (j != cpu && > is_idle_task(rq_j->curr) && rq_j->nr_running) { > + resched_curr(rq_j); > + trace_printk("IPI(%d->%d[%d]) > idle preempt\n", > + cpu, j, > rq_j->nr_running); > + } > + } > goto done; > } I'm thinking there is a more elegant solution hiding in there; possibly saving/restoring that core_cookie on the again loop should do, but I've always had the nagging suspicion that whole selection loop could be done better.