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SubjectRe: [RFC PATCH 01/10] sched: Make load tracking frequency scale-invariant
On 2 December 2014 at 15:06, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> From: Dietmar Eggemann <dietmar.eggemann@arm.com>
>
> Apply frequency scale-invariance correction factor to load tracking.
> Each segment of the sched_avg::runnable_avg_sum geometric series is now
> scaled by the current frequency so the sched_avg::load_avg_contrib of each
> entity will be invariant with frequency scaling. As a result,
> cfs_rq::runnable_load_avg which is the sum of sched_avg::load_avg_contrib,
> becomes invariant too. So the load level that is returned by
> weighted_cpuload, stays relative to the max frequency of the cpu.
>
> Then, we want the keep the load tracking values in a 32bits type, which
> implies that the max value of sched_avg::{runnable|running}_avg_sum must
> be lower than 2^32/88761=48388 (88761 is the max weight 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: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
> kernel/sched/fair.c | 28 ++++++++++++++++------------
> 1 file changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ee76d52..b41f03d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2507,9 +2507,9 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,
> int runnable,
> int running)
> {
> - u64 delta, periods;
> - u32 runnable_contrib;
> - int delta_w, decayed = 0;
> + u64 delta, scaled_delta, periods;
> + u32 runnable_contrib, scaled_runnable_contrib;
> + int delta_w, scaled_delta_w, decayed = 0;
> unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
>
> delta = now - sa->last_runnable_update;
> @@ -2543,11 +2543,12 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,
> * period and accrue it.
> */
> delta_w = 1024 - delta_w;
> + scaled_delta_w = (delta_w * scale_freq) >> SCHED_CAPACITY_SHIFT;
> +
> if (runnable)
> - sa->runnable_avg_sum += delta_w;
> + sa->runnable_avg_sum += scaled_delta_w;
> if (running)
> - sa->running_avg_sum += delta_w * scale_freq
> - >> SCHED_CAPACITY_SHIFT;
> + sa->running_avg_sum += scaled_delta_w;
> sa->avg_period += delta_w;
>
> delta -= delta_w;
> @@ -2565,20 +2566,23 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,
>
> /* Efficiently calculate \sum (1..n_period) 1024*y^i */
> runnable_contrib = __compute_runnable_contrib(periods);
> + scaled_runnable_contrib = (runnable_contrib * scale_freq)
> + >> SCHED_CAPACITY_SHIFT;
> +
> if (runnable)
> - sa->runnable_avg_sum += runnable_contrib;
> + sa->runnable_avg_sum += scaled_runnable_contrib;
> if (running)
> - sa->running_avg_sum += runnable_contrib * scale_freq
> - >> SCHED_CAPACITY_SHIFT;
> + sa->running_avg_sum += scaled_runnable_contrib;
> sa->avg_period += runnable_contrib;
> }
>
> /* Remainder of delta accrued against u_0` */
> + scaled_delta = (delta * scale_freq) >> SCHED_CAPACITY_SHIFT;
> +
> if (runnable)
> - sa->runnable_avg_sum += delta;
> + sa->runnable_avg_sum += scaled_delta;
> if (running)
> - sa->running_avg_sum += delta * scale_freq
> - >> SCHED_CAPACITY_SHIFT;
> + sa->running_avg_sum += scaled_delta;
> sa->avg_period += delta;
>
> return decayed;

Acked-by: Vincent Guittot <vincent.guittot@linaro.org>

> --
> 1.9.1
>
>


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