Re: [PATCH v9 05/10] sched: make scale_rt invariant with frequency

From: Wanpeng Li <kernellwp@gmail.com>
To: Vincent Guittot <vincent.guittot@linaro.org>,
	peterz@infradead.org, mingo@kernel.org,
	linux-kernel@vger.kernel.org, preeti@linux.vnet.ibm.com,
	Morten.Rasmussen@arm.com, kamalesh@linux.vnet.ibm.com,
	linux-arm-kernel@lists.infradead.org
Cc: riel@redhat.com, efault@gmx.de, nicolas.pitre@linaro.org,
	linaro-kernel@lists.linaro.org
Subject: Re: [PATCH v9 05/10] sched: make scale_rt invariant with frequency
Date: Tue, 25 Nov 2014 10:24:39 +0800	[thread overview]
Message-ID: <5473E867.6030607@gmail.com> (raw)
In-Reply-To: <1415033687-23294-6-git-send-email-vincent.guittot@linaro.org>

Hi Vincent,
On 11/4/14, 12:54 AM, Vincent Guittot wrote:
> The average running time of RT tasks is used to estimate the remaining compute
> capacity for CFS tasks. This remaining capacity is the original capacity scaled
> down by a factor (aka scale_rt_capacity). This estimation of available capacity
> must also be invariant with frequency scaling.
>
> A frequency scaling factor is applied on the running time of the RT tasks for
> computing scale_rt_capacity.
>
> In sched_rt_avg_update, we scale the RT execution time like below:
> rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT
>
> Then, scale_rt_capacity can be summarized by:
> scale_rt_capacity = SCHED_CAPACITY_SCALE -
> 		((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)

The 'period' aka 'total' in the scale_rt_capacity(), why it is 
sched_avg_period() + delta instead of sched_avg_period()?

Regards,
Wanpeng Li

>
> We can optimize by removing right and left shift in the computation of rq->rt_avg
> and scale_rt_capacity
>
> The call to arch_scale_frequency_capacity in the rt scheduling path might be
> a concern for RT folks because I'm not sure whether we can rely on
> arch_scale_freq_capacity to be short and efficient ?
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>   kernel/sched/fair.c  | 17 +++++------------
>   kernel/sched/sched.h |  4 +++-
>   2 files changed, 8 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a5039da..b37c27b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5785,7 +5785,7 @@ unsigned long __weak arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
>   static unsigned long scale_rt_capacity(int cpu)
>   {
>   	struct rq *rq = cpu_rq(cpu);
> -	u64 total, available, age_stamp, avg;
> +	u64 total, used, age_stamp, avg;
>   	s64 delta;
>   
>   	/*
> @@ -5801,19 +5801,12 @@ static unsigned long scale_rt_capacity(int cpu)
>   
>   	total = sched_avg_period() + delta;
>   
> -	if (unlikely(total < avg)) {
> -		/* Ensures that capacity won't end up being negative */
> -		available = 0;
> -	} else {
> -		available = total - avg;
> -	}
> +	used = div_u64(avg, total);
>   
> -	if (unlikely((s64)total < SCHED_CAPACITY_SCALE))
> -		total = SCHED_CAPACITY_SCALE;
> +	if (likely(used < SCHED_CAPACITY_SCALE))
> +		return SCHED_CAPACITY_SCALE - used;
>   
> -	total >>= SCHED_CAPACITY_SHIFT;
> -
> -	return div_u64(available, total);
> +	return 1;
>   }
>   
>   static void update_cpu_capacity(struct sched_domain *sd, int cpu)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index c34bd11..fc5b152 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1312,9 +1312,11 @@ static inline int hrtick_enabled(struct rq *rq)
>   
>   #ifdef CONFIG_SMP
>   extern void sched_avg_update(struct rq *rq);
> +extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
> +
>   static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
>   {
> -	rq->rt_avg += rt_delta;
> +	rq->rt_avg += rt_delta * arch_scale_freq_capacity(NULL, cpu_of(rq));
>   	sched_avg_update(rq);
>   }
>   #else