linux-kernel - Re: [RFC PATCH 4/6] sched/dl: Improve capacity-aware wakeup

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Date:   Wed, 8 May 2019 11:08:55 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Luca Abeni <luca.abeni@...tannapisa.it>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Quentin Perret <quentin.perret@....com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [RFC PATCH 4/6] sched/dl: Improve capacity-aware wakeup

On 06/05/19 06:48, Luca Abeni wrote:
> From: luca abeni <luca.abeni@...tannapisa.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@...tannapisa.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'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)?

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