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Date: Thu, 30 Mar 2017 04:21:08 -0700
From: Paul Turner <pjt@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Yuyang Du <yuyang.du@...el.com>, Ingo Molnar <mingo@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Benjamin Segall <bsegall@...gle.com>,
Morten Rasmussen <morten.rasmussen@....com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Matt Fleming <matt@...eblueprint.co.uk>,
umgwanakikbuti@...il.com
Subject: Re: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched_avg()
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2767,7 +2767,7 @@ static const u32 __accumulated_sum_N32[]
> * Approximate:
> * val * y^n, where y^32 ~= 0.5 (~1 scheduling period)
> */
> -static __always_inline u64 decay_load(u64 val, u64 n)
> +static u64 decay_load(u64 val, u64 n)
> {
> unsigned int local_n;
>
> @@ -2795,32 +2795,113 @@ static __always_inline u64 decay_load(u6
> return val;
> }
>
> -/*
> - * For updates fully spanning n periods, the contribution to runnable
> - * average will be: \Sum 1024*y^n
> - *
> - * We can compute this reasonably efficiently by combining:
> - * y^PERIOD = 1/2 with precomputed \Sum 1024*y^n {for n <PERIOD}
> - */
> -static u32 __compute_runnable_contrib(u64 n)
> +static u32 __accumulate_sum(u64 periods, u32 period_contrib, u32 remainder)
- The naming here is really ambiguous:
"__accumulate_sum" -> "__accumulate_pelt_segments"?
- Passing in "remainder" seems irrelevant to the sum accumulation. It would be
more clear to handle it from the caller.
>
> {
> - u32 contrib = 0;
> + u32 c1, c2, c3 = remainder; /* y^0 == 1 */
>
> - if (likely(n <= LOAD_AVG_PERIOD))
> - return runnable_avg_yN_sum[n];
> - else if (unlikely(n >= LOAD_AVG_MAX_N))
> + if (!periods)
> + return remainder - period_contrib;
This is super confusing. It only works because remainder already had
period_contrib aggregated _into_ it. We're literally computing:
remainder + period_contrib - period_contrib
We should just not call this in the !periods case and handle the remainder
below.
> +
> + if (unlikely(periods >= LOAD_AVG_MAX_N))
> return LOAD_AVG_MAX;
>
> - /* Since n < LOAD_AVG_MAX_N, n/LOAD_AVG_PERIOD < 11 */
> - contrib = __accumulated_sum_N32[n/LOAD_AVG_PERIOD];
> - n %= LOAD_AVG_PERIOD;
> - contrib = decay_load(contrib, n);
> - return contrib + runnable_avg_yN_sum[n];
> + /*
> + * c1 = d1 y^(p+1)
> + */
> + c1 = decay_load((u64)(1024 - period_contrib), periods);
> +
> + periods -= 1;
> + /*
> + * For updates fully spanning n periods, the contribution to runnable
> + * average will be:
> + *
> + * c2 = 1024 \Sum y^n
> + *
> + * We can compute this reasonably efficiently by combining:
> + *
> + * y^PERIOD = 1/2 with precomputed 1024 \Sum y^n {for: n < PERIOD}
> + */
> + if (likely(periods <= LOAD_AVG_PERIOD)) {
> + c2 = runnable_avg_yN_sum[periods];
> + } else {
> + c2 = __accumulated_sum_N32[periods/LOAD_AVG_PERIOD];
This still wants the comment justifying why we can't overflow.
> + periods %= LOAD_AVG_PERIOD;
> + c2 = decay_load(c2, periods);
> + c2 += runnable_avg_yN_sum[periods];
> + }
> +
> + return c1 + c2 + c3;
> }
>
> #define cap_scale(v, s) ((v)*(s) >> SCHED_CAPACITY_SHIFT)
>
> /*
> + * Accumulate the three separate parts of the sum; d1 the remainder
> + * of the last (incomplete) period, d2 the span of full periods and d3
> + * the remainder of the (incomplete) current period.
> + *
> + * d1 d2 d3
> + * ^ ^ ^
> + * | | |
> + * |<->|<----------------->|<--->|
> + * ... |---x---|------| ... |------|-----x (now)
> + *
> + * p
> + * u' = (u + d1) y^(p+1) + 1024 \Sum y^n + d3 y^0
> + * n=1
> + *
> + * = u y^(p+1) + (Step 1)
> + *
> + * p
> + * d1 y^(p+1) + 1024 \Sum y^n + d3 y^0 (Step 2)
> + * n=1
> + */
> +static __always_inline u32
> +accumulate_sum(u64 delta, int cpu, struct sched_avg *sa,
> + unsigned long weight, int running, struct cfs_rq *cfs_rq)
> +{
> + unsigned long scale_freq, scale_cpu;
> + u64 periods;
> + u32 contrib;
> +
> + 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) */
> +
> + if (periods) {
> + sa->load_sum = decay_load(sa->load_sum, periods);
> + if (cfs_rq) {
> + cfs_rq->runnable_load_sum =
> + decay_load(cfs_rq->runnable_load_sum, periods);
> + }
> + sa->util_sum = decay_load((u64)(sa->util_sum), periods);
Move step 2 here, handle remainder below.
> + }
> +
> + /*
> + * Step 2
> + */
> + delta %= 1024;
> + contrib = __accumulate_sum(periods, sa->period_contrib, delta);
> + sa->period_contrib = delta;
> +
> + contrib = cap_scale(contrib, scale_freq);
> + if (weight) {
> + sa->load_sum += weight * contrib;
Is this correct in the iterated periods > LOAD_AVG_MAX_N case?
I don't think the decay above is guaranteed to return these to zero.
> + if (cfs_rq)
> + cfs_rq->runnable_load_sum += weight * contrib;
> + }
> + if (running)
> + sa->util_sum += contrib * scale_cpu;
> +
> + return periods;
> +}
> +
> +/*
> * We can represent the historical contribution to runnable average as the
> * coefficients of a geometric series. To do this we sub-divide our runnable
> * history into segments of approximately 1ms (1024us); label the segment that
> @@ -2852,10 +2933,7 @@ 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)
> {
> - u64 delta, scaled_delta, periods;
> - u32 contrib;
> - unsigned int delta_w, scaled_delta_w, decayed = 0;
> - unsigned long scale_freq, scale_cpu;
> + u64 delta;
>
> delta = now - sa->last_update_time;
> /*
> @@ -2876,81 +2954,27 @@ ___update_load_avg(u64 now, int cpu, str
> return 0;
> sa->last_update_time = now;
>
> - scale_freq = arch_scale_freq_capacity(NULL, cpu);
> - scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
> -
> - /* delta_w is the amount already accumulated against our next period */
> - delta_w = sa->period_contrib;
> - if (delta + delta_w >= 1024) {
> - decayed = 1;
> -
> - /* how much left for next period will start over, we don't know yet */
> - sa->period_contrib = 0;
> -
> - /*
> - * Now that we know we're crossing a period boundary, figure
> - * out how much from delta we need to complete the current
> - * period and accrue it.
> - */
> - delta_w = 1024 - delta_w;
> - scaled_delta_w = cap_scale(delta_w, scale_freq);
> - if (weight) {
> - sa->load_sum += weight * scaled_delta_w;
> - if (cfs_rq) {
> - cfs_rq->runnable_load_sum +=
> - weight * scaled_delta_w;
> - }
> - }
> - if (running)
> - sa->util_sum += scaled_delta_w * scale_cpu;
> -
> - delta -= delta_w;
> -
> - /* Figure out how many additional periods this update spans */
> - periods = delta / 1024;
> - delta %= 1024;
> -
> - sa->load_sum = decay_load(sa->load_sum, periods + 1);
> - if (cfs_rq) {
> - cfs_rq->runnable_load_sum =
> - decay_load(cfs_rq->runnable_load_sum, periods + 1);
> - }
> - sa->util_sum = decay_load((u64)(sa->util_sum), periods + 1);
> -
> - /* Efficiently calculate \sum (1..n_period) 1024*y^i */
> - contrib = __compute_runnable_contrib(periods);
> - contrib = cap_scale(contrib, scale_freq);
> - if (weight) {
> - sa->load_sum += weight * contrib;
> - if (cfs_rq)
> - cfs_rq->runnable_load_sum += weight * contrib;
> - }
> - if (running)
> - sa->util_sum += contrib * scale_cpu;
> - }
> -
> - /* Remainder of delta accrued against u_0` */
> - scaled_delta = cap_scale(delta, scale_freq);
> - if (weight) {
> - sa->load_sum += weight * scaled_delta;
> - if (cfs_rq)
> - cfs_rq->runnable_load_sum += weight * scaled_delta;
> - }
> - if (running)
> - sa->util_sum += scaled_delta * scale_cpu;
> -
> - sa->period_contrib += delta;
> + /*
> + * Now we know we crossed measurement unit boundaries. The *_avg
> + * accrues by two steps:
> + *
> + * 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))
> + return 0;
>
> - if (decayed) {
> - sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX);
> - if (cfs_rq) {
> - cfs_rq->runnable_load_avg =
> - div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX);
> - }
> - sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
> + /*
> + * Step 2: update *_avg.
> + */
> + sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX);
> + if (cfs_rq) {
> + cfs_rq->runnable_load_avg =
> + div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX);
> }
> + sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
>
> - return decayed;
> + return 1;
> }
>
> static int
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