Re: [PATCHv4 12/12] sched/core: Disable SD_PREFER_SIBLING on asymmetric cpu capacity domains

[Vincent Guittot]

Hi Valentin,

On Tue, 31 Jul 2018 at 14:33, Valentin Schneider
<valentin.schneider@xxxxxxx> wrote:
>
> Hi,
>
> On 31/07/18 13:17, Vincent Guittot wrote:
> > On Fri, 6 Jul 2018 at 16:31, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
> >>
> >> On Fri, Jul 06, 2018 at 12:18:17PM +0200, Vincent Guittot wrote:
> >>> [...]
> >>
> >> Scheduling one task per cpu when n_task == n_cpus on asymmetric
> >> topologies is generally broken already and this patch set doesn't fix
> >> that problem.
> >>
> >> SD_PREFER_SIBLING might seem to help in very specific cases:
> >> n_litte_cpus == n_big_cpus. In that case the little group might
> >> classified as overloaded. It doesn't guarantee that anything gets pulled
> >> as the grp_load/grp_capacity in the imbalance calculation on some system
> >> still says the little cpus are more loaded than the bigs despite one of
> >> them being idle. That depends on the little cpu capacities.
> >>
> >> On systems where n_little_cpus != n_big_cpus SD_PREFER_SIBLING is broken
> >> as it assumes the group_weight to be the same. This is the case on Juno
> >> and several other platforms.
> >>
> >> IMHO, SD_PREFER_SIBLING isn't the solution to this problem. It might
> >
> > I agree but this patchset creates a regression in the scheduling behavior
> >
> >> help for a limited subset of topologies/capacities but the right
> >> solution is to change the imbalance calculation. As the name says, it is
> >
> > Yes that what does the prototype that I came with.
> >
> >> meant to spread tasks and does so unconditionally. For asymmetric
> >> systems we would like to consider cpu capacity before migrating tasks.
> >>
> >>> When running the tests of your cover letter, 1 long
> >>> running task is often co scheduled on a big core whereas short pinned
> >>> tasks are still running and a little core is idle which is not an
> >>> optimal scheduling decision
> >>
> >> This can easily happen with SD_PREFER_SIBLING enabled too so I wouldn't
> >> say that this patch breaks anything that isn't broken already. In fact
> >> we this happening with and without this patch applied.
> >
> > At least for the use case  above, this doesn't happen when
> > SD_PREFER_SIBLING is set
> >
>
> On my HiKey960 I can see coscheduling on a big CPU while a LITTLE is free
> with **and** without SD_PREFER_SIBLING. Having it set only means that in
> some cases the imbalance will be re-classified as group_overloaded instead
> of group_misfit_task, so we'll skip the misfit logic when we shouldn't (this
> happens on Juno for instance).

Can you give more details about your test case ?

>
> It does nothing for the "1 task per CPU" problem that Morten described above.
> When you have this little amount of tasks, load isn't very relevant, but it
> skews the load-balancer into thinking the LITTLE CPUs are more busy than
> the bigs even though there's an idle one in the lot.
>
> >>
> >> Morten