linux-kernel - Re: [PATCH RESEND v9 04/10] sched: Make sched entity usage tracking scale-invariant

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Date:	Thu, 19 Feb 2015 17:34:42 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com,
	kamalesh@...ux.vnet.ibm.com, riel@...hat.com, efault@....de,
	nicolas.pitre@...aro.org, dietmar.eggemann@....com,
	linaro-kernel@...ts.linaro.org, Paul Turner <pjt@...gle.com>,
	Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH RESEND v9 04/10] sched: Make sched entity usage tracking
 scale-invariant

On Thu, Jan 15, 2015 at 11:09:24AM +0100, Vincent Guittot wrote:
> From: Morten Rasmussen <morten.rasmussen@....com>
> 
> Apply frequency scale-invariance correction factor to usage tracking.
> Each segment of the running_load_avg geometric series is now scaled by the
> current frequency so the utilization_avg_contrib of each entity will be
> invariant with frequency scaling. 

> As a result, utilization_load_avg which is
> the sum of utilization_avg_contrib, becomes invariant too. So the usage level
> that is returned by get_cpu_usage, stays relative to the max frequency as the
> cpu_capacity which is is compared against.

> Then, we want the keep the load tracking values in a 32bits type, which implies
> that the max value of {runnable|running}_avg_sum must be lower than
> 2^32/88761=48388 (88761 is the max weigth of a task). 

> As LOAD_AVG_MAX = 47742,
> arch_scale_freq_capacity must return a value less than
> (48388/47742) << SCHED_CAPACITY_SHIFT = 1037 (SCHED_SCALE_CAPACITY = 1024).
> So we define the range to [0..SCHED_SCALE_CAPACITY] in order to avoid overflow.

> cc: Paul Turner <pjt@...gle.com>
> cc: Ben Segall <bsegall@...gle.com>
> 
> Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a96affd..a5039da 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2266,6 +2266,8 @@ static u32 __compute_runnable_contrib(u64 n)
>  	return contrib + runnable_avg_yN_sum[n];
>  }
>  
> +unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu);

There is no actual function definition in this patch; this would hinder
linking, no?


>  		if (running)
> +			sa->running_avg_sum += delta_w * scale_freq
> +				>> SCHED_CAPACITY_SHIFT;


>  		if (running)
> +			sa->running_avg_sum += runnable_contrib * scale_freq
> +				>> SCHED_CAPACITY_SHIFT;

>  	if (running)
> +		sa->running_avg_sum += delta * scale_freq
> +			>> SCHED_CAPACITY_SHIFT;

If you're going to respin; please but {} around that.
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