Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH 7/8] sched/fair: Attempt misfit active balance when migration_type != migrate_misfit | Date | Tue, 09 Feb 2021 18:19:43 +0000 |
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On 09/02/21 09:58, Vincent Guittot wrote: > On Thu, 28 Jan 2021 at 19:32, Valentin Schneider >> Giving group_misfit_task a higher group_classify() priority than >> group_imbalance doesn't seem like the right thing to do. Instead, make > > Could you explain why ? >
Morten had intentionally placed it above (then) group_other but below group_imbalanced:
3b1baa6496e6 ("sched/fair: Add 'group_misfit_task' load-balance type")
The reasoning being misfit balance shouldn't take higher priority than jarring imbalance issues. group_imbalanced is a mixed bag and difficult to classify, but for sure group_overloaded takes priority as it ought to imply you can move tasks around without doing an active balance (there's more tasks than CPUs).
Then again, we do have issues where the busiest group is group_overloaded, but we'd "want" this to be classified as misfit. This ties in with patch 8.
Take the CPU hog vs pcpu kworker example on a big.LITTLE platform:
a,b,c,d are our CPU-hogging tasks k is a per-CPU kworker
{CPU0 | a a a a k LITTLE{CPU1 | b b b b a ------|--------- {CPU2 | c c c c . bigs {CPU3 | d d d d ^ | | newidle pull
CPU2 finished its work and goes through a newidle balance. Ideally here it would pull either 'a' or 'b' which are CPU-bound tasks running on LITTLE CPUs. Unfortunately, a per-CPU kworker woke up on CPU0, so since we have:
DIE [ ] MC [ ][ ] 0 1 2 3
the DIE (0-1) sched_group has 3 runnable tasks, two of which are CPU hogs: it gets classified as group_overloaded. Only task 'a' can be pulled, and it requires patch 8 to be migrated in this scenario.
I'm not sure how we could better classify this, even if admitting we started tracking preempted misfit tasks. Perhaps not group classification itself, but the migration_type? i.e. something like
if (nr_running - sgs->group_weight <= nr_misfits) => all preempted tasks are misfit => migration_type = migrate_misfit
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