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SubjectRe: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched_avg()
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On 31/03/17 12:23, Peter Zijlstra wrote:
> On Fri, Mar 31, 2017 at 02:58:57AM -0700, Paul Turner wrote:
>>> So lets pull it out again -- but I don't think we need to undo all of
>>> yuyang's patches for that. So please, look at the patch I proposed for
>>> the problem you spotted. Lets fix the current state and take it from
>>> there, ok?
>>
>> Oh, absolutely. I'm not really not proposing re-vulcanizing any
>> rubber here. Just saying that this particular problem is just one
>> facet of the weight mixing. 100% agree on fixing this as is and
>> iterating. I was actually thinking that these changes (which really
>> nicely simplify the calculation) greatly simplify restoring the
>> separation there.
>
> So on top of the previous patch; I've arrived (without testing and
> considering numerical overflow much) at the following patch.

I did some testing (performance gov, no big.little) with this and for a
single task the load_avg values are much higher now on 64bit because of
the missing scale_load_down on weight.
Are you planning to get rid of scale_load_down() for weight because of
the correctness problems of the signal mentioned by Paul in this thread?

"... c) It doesn't work with scale_load_down and fractional shares below
SCHED_LOAD_SCALE [we multiply in a zero -> zero rq load] ... "

>
> Its known broken for things like update_cfs_rq_load_avg() where doing
> this will unconditionally flatten load_sum, which seems undesired. Need
> to think more on this.
>
> Also, I've pulled out the cpufreq scaling, which I'm not sure is the
> right thing to do. Vincent wants to scale time, which would be
> persistent in the history, unlike this now.

This patch moves freq and cpu from accumulate_sum() to ___update_load_avg()?

[...]

> @@ -2820,15 +2820,11 @@ static u32 __accumulate_pelt_segments(u6
> */
> static __always_inline u32
> accumulate_sum(u64 delta, int cpu, struct sched_avg *sa,
> - unsigned long weight, int running, struct cfs_rq *cfs_rq)
> + bool load, bool running, struct cfs_rq *cfs_rq)
> {
> - unsigned long scale_freq, scale_cpu;
> - u32 contrib = (u32)delta; /* p == 0 -> delta < 1024 */
> + u32 contrib = (u32)delta;
> u64 periods;
>
> - scale_freq = arch_scale_freq_capacity(NULL, cpu);
> - scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
> -
> delta += sa->period_contrib;
> periods = delta / 1024; /* A period is 1024us (~1ms) */
>
> @@ -2852,14 +2848,13 @@ accumulate_sum(u64 delta, int cpu, struc
> }
> sa->period_contrib = delta;
>
> - contrib = cap_scale(contrib, scale_freq);
> - if (weight) {
> - sa->load_sum += weight * contrib;
> + if (load) {
> + sa->load_sum += contrib;
> if (cfs_rq)
> - cfs_rq->runnable_load_sum += weight * contrib;
> + cfs_rq->runnable_load_sum += contrib;
> }
> if (running)
> - sa->util_sum += contrib * scale_cpu;
> + sa->util_sum += contrib;
>
> return periods;
> }
> @@ -2894,9 +2889,10 @@ accumulate_sum(u64 delta, int cpu, struc
> */
> static __always_inline int
> ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
> - unsigned long weight, int running, struct cfs_rq *cfs_rq)
> + unsigned long weight, bool running, struct cfs_rq *cfs_rq)
> {
> - u64 delta;
> + unsigned long scale_freq, scale_cpu;
> + u64 delta, load;
>
> delta = now - sa->last_update_time;
> /*
> @@ -2924,18 +2920,22 @@ ___update_load_avg(u64 now, int cpu, str
> * Step 1: accumulate *_sum since last_update_time. If we haven't
> * crossed period boundaries, finish.
> */
> - if (!accumulate_sum(delta, cpu, sa, weight, running, cfs_rq))
> + if (!accumulate_sum(delta, cpu, sa, !!weight, running, cfs_rq))
> return 0;
>
> + scale_freq = arch_scale_freq_capacity(NULL, cpu);
> + scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
> +
> /*
> * Step 2: update *_avg.
> */
> - sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX);
> + load = cap_scale(weight, scale_freq);
> + sa->load_avg = div_u64(load * sa->load_sum, LOAD_AVG_MAX);
> if (cfs_rq) {
> cfs_rq->runnable_load_avg =
> - div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX);
> + div_u64(load * cfs_rq->runnable_load_sum, LOAD_AVG_MAX);
> }
> - sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
> + sa->util_avg = (cap_scale(scale_cpu, scale_freq) * sa->util_sum) / LOAD_AVG_MAX;
>
> return 1;
> }

[...]

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