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SubjectRe: [RFC PATCH 4/6] sched/dl: Improve capacity-aware wakeup
On Wed, 8 May 2019 11:08:55 +0200
Juri Lelli <juri.lelli@redhat.com> wrote:

> On 06/05/19 06:48, Luca Abeni wrote:
> > From: luca abeni <luca.abeni@santannapisa.it>
> >
> > Instead of considering the "static CPU bandwidth" allocated to
> > a SCHED_DEADLINE task (ratio between its maximum runtime and
> > reservation period), try to use the remaining runtime and time
> > to scheduling deadline.
> >
> > Signed-off-by: luca abeni <luca.abeni@santannapisa.it>
> > ---
> > kernel/sched/cpudeadline.c | 9 +++++++--
> > 1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> > index d21f7905b9c1..111dd9ac837b 100644
> > --- a/kernel/sched/cpudeadline.c
> > +++ b/kernel/sched/cpudeadline.c
> > @@ -114,8 +114,13 @@ static inline int dl_task_fit(const struct
> > sched_dl_entity *dl_se, int cpu, u64 *c)
> > {
> > u64 cap = (arch_scale_cpu_capacity(NULL, cpu) *
> > arch_scale_freq_capacity(cpu)) >> SCHED_CAPACITY_SHIFT;
> > - s64 rel_deadline = dl_se->dl_deadline;
> > - u64 rem_runtime = dl_se->dl_runtime;
> > + s64 rel_deadline = dl_se->deadline -
> > sched_clock_cpu(smp_processor_id());
> > + u64 rem_runtime = dl_se->runtime;
> > +
> > + if ((rel_deadline < 0) || (rel_deadline *
> > dl_se->dl_runtime < dl_se->dl_deadline * rem_runtime)) {
> > + rel_deadline = dl_se->dl_deadline;
> > + rem_runtime = dl_se->dl_runtime;
> > + }
>
> So, are you basically checking if current remaining bw can be consumed
> safely?

I check if the current runtime (rescaled based on the capacity) is
smaller than the time to the current scheduling deadline (basically, if
it can be consumed in time).

However, if
q / (d - t) > Q / P
(where "q" is the current runtime, "d" is the scheduling deadline, "Q"
is the maximum runtime, and "P" is the CBS period), then a new
scheduling deadline will be generated (later), and the runtime will be
reset to Q... So, I need to use the maximum budget and CBS period for
checking if the task fits in the core.

>
> I'm not actually sure if looking at dynamic values is what we need to
> do at this stage. By considering static values we fix admission
> control (and scheduling). Aren't dynamic values more to do with
> energy tradeoffs (and so to be introduced when starting to look at
> the energy model)?

Using the current runtime and scheduling deadline might allow to
migrate a task to SMALL cores (if its remaining runtime is small
enough), even if the rescaled Q is larger than P.
So, in theory it might allow to reduce the load on big cores.

If we decide that this is overkilling, I can just drop the patch.



Luca

> Another pair of hands maybe is to look at the dynamic spare bw of CPUs
> (to check that we don't overload CPUs).
>
> Thanks,
>
> - Juri

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