linux-kernel - Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity

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Date:   Wed, 31 Aug 2016 03:31:07 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Steve Muckle <steve.muckle@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Steve Muckle <smuckle@...aro.org>
Subject: Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity

On Friday, August 26, 2016 11:40:48 AM Steve Muckle wrote:
> A policy of going to fmax on any RT activity will be detrimental
> for power on many platforms. Often RT accounts for only a small amount
> of CPU activity so sending the CPU frequency to fmax is overkill. Worse
> still, some platforms may not be able to even complete the CPU frequency
> change before the RT activity has already completed.
> 
> Cpufreq governors have not treated RT activity this way in the past so
> it is not part of the expected semantics of the RT scheduling class. The
> DL class offers guarantees about task completion and could be used for
> this purpose.
> 
> Modify the schedutil algorithm to instead use rt_avg as an estimate of
> RT utilization of the CPU.
> 
> Based on previous work by Vincent Guittot <vincent.guittot@...aro.org>.

If we do it for RT, why not to do a similar thing for DL?  As in the
original patch from Peter, for example?

> Signed-off-by: Steve Muckle <smuckle@...aro.org>
> ---
>  kernel/sched/cpufreq_schedutil.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index cb8a77b1ef1b..89094a466250 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -146,13 +146,21 @@ static unsigned int get_next_freq(struct sugov_cpu *sg_cpu, unsigned long util,
>  
>  static void sugov_get_util(unsigned long *util, unsigned long *max)
>  {
> -	struct rq *rq = this_rq();
> -	unsigned long cfs_max;
> +	int cpu = smp_processor_id();
> +	struct rq *rq = cpu_rq(cpu);
> +	unsigned long max_cap, rt;
> +	s64 delta;
>  
> -	cfs_max = arch_scale_cpu_capacity(NULL, smp_processor_id());
> +	max_cap = arch_scale_cpu_capacity(NULL, cpu);
>  
> -	*util = min(rq->cfs.avg.util_avg, cfs_max);
> -	*max = cfs_max;
> +	delta = rq_clock(rq) - rq->age_stamp;
> +	if (unlikely(delta < 0))
> +		delta = 0;
> +	rt = div64_u64(rq->rt_avg, sched_avg_period() + delta);
> +	rt = (rt * max_cap) >> SCHED_CAPACITY_SHIFT;

These computations are rather heavy, so I wonder if they are avoidable based
on the flags, for example?

Plus is SCHED_CAPACITY_SHIFT actually defined for all architectures?

One more ugly thing is about using rq_clock(rq) directly from here whereas we
pass it around as the 'time' argument elsewhere.

> +
> +	*util = min(rq->cfs.avg.util_avg + rt, max_cap);
> +	*max = max_cap;
>  }

Thanks,
Rafael