Messages in this thread | | | Subject | Re: [PATCH] nohz_full: Make sched_should_stop_tick() more conservative | From | Rik van Riel <> | Date | Mon, 04 Apr 2016 15:36:11 -0400 |
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On Mon, 2016-04-04 at 15:31 -0400, Chris Metcalf wrote: > On 4/4/2016 3:12 PM, Rik van Riel wrote: > > > > On Fri, 2016-04-01 at 15:42 -0400, Chris Metcalf wrote: > > > > > > On arm64, when calling enqueue_task_fair() from > > > migration_cpu_stop(), > > > we find the nr_running value updated by add_nr_running(), but the > > > cfs.nr_running value has not always yet been > > > updated. Accordingly, > > > the sched_can_stop_tick() false returns true when we are > > > migrating a > > > second task onto a core. > > I don't get it. > > > > Looking at the enqueue_task_fair(), I see this: > > > > for_each_sched_entity(se) { > > cfs_rq = cfs_rq_of(se); > > cfs_rq->h_nr_running++; > > ... > > } > > > > if (!se) > > add_nr_running(rq, 1); > > > > What is the difference between cfs_rq->h_nr_running, > > and rq->cfs.nr_running? > > > > Why do we have two? > > Are we simply testing against the wrong one in > > sched_can_stop_tick? > It seems that using the non-CFS one is what we want. I don't know > whether > using a different CFS count instead might be more correct. > > Since I'm not sure what causes the difference I see between tile > (correct) > and arm64 (incorrect) it's hard for me to speculate. > > > > > > > > > Correct this by using rq->nr_running instead of rq- > > > >cfs.nr_running. > > > This should always be more conservative, and reverts the test to > > > the > > > form it had before commit 76d92ac305f2 ("sched: Migrate sched to > > > use > > > new tick dependency mask model"). > > That would cause us to run the timer tick while running > > a single SCHED_RR real time task, with a single > > SCHED_OTHER task sitting in the background (which will > > not get run until the SCHED_RR task is done). > No, because in sched_can_stop_tick(), we first handle the special > cases of RR or FIFO tasks present. For example, RR: > > if (rq->rt.rr_nr_running) { > if (rq->rt.rr_nr_running == 1) > return true; > else > return false; > } > > Once we see there's any RR tasks running, the return value > ignores any possible SCHED_OTHER tasks. Only after the code > concludes there are no RR/FIFO tasks do we even look at > the over nr_running value.
OK, fair enough. I guess both of the RT cases are covered already.
Patch gets my:
Acked-by: Rik van Riel <riel@redhat.com>
-- All Rights Reversed.
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