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



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