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Date:   Wed, 8 May 2019 11:24:37 +0200
From:   luca abeni <luca.abeni@santannapisa.it>
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        "Paul E . McKenney" <paulmck@linux.ibm.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Quentin Perret <quentin.perret@arm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>
Subject: Re: [RFC PATCH 4/6] sched/dl: Improve capacity-aware wakeup
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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