Re: Scheduling tasks on idle cpu

[Vincent Guittot]

On Thu, 14 Apr 2022 at 01:57, Qais Yousef <qais.yousef@xxxxxxx> wrote:
>
> On 04/12/22 11:07, Vincent Guittot wrote:
> > On Tue, 12 Apr 2022 at 10:39, David Laight <David.Laight@xxxxxxxxxx> wrote:
> > > Yes I want the CFS scheduler to pick an idle cpu in preference
> > > to an active RT one.
> >
> > When task 34512 wakes up, scheduler checks if prev or this cpu are
> > idle which is not the case for you. Then, it compares the load of prev
> > and this_cpu and seems to select this_cpu (cpu17).
> >
> > Once cpu17 selected, it will try to find an idle cpu which shares LLC
> > but it seems that the scheduler didn't find one and finally keeps task
> > 34512 on this_cpu.
> >
> > Note that during the next tick, a load balance will be trigger if
> > this_cpu still have both RT and task 34512,
>
> David said there are idle cpus
>
>         "  There are two physical cpu with 20 cores each (with hyperthreading).
>         16, 18, 34, 36 and 38 were idle. So both 16 and 18 should be on the
>         same NUMA node. All the others are running the same RT thread code.  "
>
> Except for the possibility of them becoming idle just after the task has woken
> up, shouldn't one of them have been picked?

we don't loop on all cpus in the LLC to find an idle one but compute a
reasonable number of iteration based on the avg_idle

David can rerun is use case after disabling sched_feat(SIS_PROP)

>
> Thanks
>
> --
> Qais Yousef