Messages in this thread | | | Date | Wed, 12 Feb 2020 16:11:03 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH 1/4] sched/fair: reorder enqueue/dequeue_task_fair path |
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On Wed, Feb 12, 2020 at 03:47:30PM +0100, Vincent Guittot wrote: > > I'm having trouble reconciling the patch with the description and the > > comments explaining the intent behind the code are unhelpful. > > > > There are two loops before and after your patch -- the first dealing with > > sched entities that are not on a runqueue and the second for the remaining > > entities that are. The intent appears to be to update the load averages > > once the entity is active on a runqueue. > > > > I'm not getting why the changelog says everything related to cfs is > > now done in one loop because there are still two. But even if you do > > get throttled, it's not clear why you jump to the !se check given that > > for_each_sched_entity did not complete. What it *does* appear to do is > > have all the h_nr_running related to entities being enqueued updated in > > one loop and all remaining entities stats updated in the other. > > Let's take the example of 2 levels in addition to root so we have : > root->cfs1->cfs2 > Now we enqueue a task T1 on cfs2 but cfs1 is throttled, we will have > the sequence: > > In 1st for_each_sched_entity loop: > loop 1 > enqueue_entity (T1->se, cfs2) which calls update load_avg(cfs2) > cfs2->h_nr_running++; > loop 2 > enqueue_entity (cfs2->gse, cfs1) which calls update load_avg(cfs1) > break because cfs1 is throttled > > In 2nd for_each_sched_entity loop: > loop 1 > cfs1->h_nr_running++ > break because throttled > > Using the 2nd loop for incrementing h_nr_running of the throttled cfs > is useless and we could do that directly in 1st loop and skip the 2nd > loop > > With this patch we have : > > In 1st for_each_sched_entity loop: > loop 1 > enqueue_entity (T1->se, cfs2) which update load_avg(cfs2) > cfs2->h_nr_running++; > loop 2 > enqueue_entity (cfs2->gse, cfs1) which update load_avg(cfs1) > cfs1->h_nr_running++ > skip the 2nd for_each_sched_entity entirely > > Then the patch also reorders the call to update_load_avg() and the > increment of h_nr_running > > Before the patch we had different order between the to > for_each_sched_entity which is not a problem because there is > currently no relation between both. But the following patches make > PELT using h_nr_running so we must have the same ordering to prevent > updating pelt with the wrong h_nr_running value >
Ok, understood. Thanks for clearing that up!
-- Mel Gorman SUSE Labs
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