summaryrefslogtreecommitdiffstats
diff options
context:
space:
mode:
authorYuyang Du <yuyang.du@intel.com>2017-02-13 05:44:23 +0800
committerIngo Molnar <mingo@kernel.org>2017-03-30 09:43:41 +0200
commita481db34b9beb7a9647c23f2320dd38a2b1d681f (patch)
treeda9a81b164c13cd3d8577c41b658b768ec326230
parent0ccb977f4c80b921a8bf6a2c4b8ea0c1fed6553c (diff)
downloadlinux-a481db34b9beb7a9647c23f2320dd38a2b1d681f.tar.gz
linux-a481db34b9beb7a9647c23f2320dd38a2b1d681f.tar.bz2
linux-a481db34b9beb7a9647c23f2320dd38a2b1d681f.zip
sched/fair: Optimize ___update_sched_avg()
The main PELT function ___update_load_avg(), which implements the accumulation and progression of the geometric average series, is implemented along the following lines for the scenario where the time delta spans all 3 possible sections (see figure below): 1. add the remainder of the last incomplete period 2. decay old sum 3. accumulate new sum in full periods since last_update_time 4. accumulate the current incomplete period 5. update averages Or: d1 d2 d3 ^ ^ ^ | | | |<->|<----------------->|<--->| ... |---x---|------| ... |------|-----x (now) load_sum' = (load_sum + weight * scale * d1) * y^(p+1) + (1,2) p weight * scale * 1024 * \Sum y^n + (3) n=1 weight * scale * d3 * y^0 (4) load_avg' = load_sum' / LOAD_AVG_MAX (5) Where: d1 - is the delta part completing the remainder of the last incomplete period, d2 - is the delta part spannind complete periods, and d3 - is the delta part starting the current incomplete period. We can simplify the code in two steps; the first step is to separate the first term into new and old parts like: (load_sum + weight * scale * d1) * y^(p+1) = load_sum * y^(p+1) + weight * scale * d1 * y^(p+1) Once we've done that, its easy to see that all new terms carry the common factors: weight * scale If we factor those out, we arrive at the form: load_sum' = load_sum * y^(p+1) + weight * scale * (d1 * y^(p+1) + p 1024 * \Sum y^n + n=1 d3 * y^0) Which results in a simpler, smaller and faster implementation. Signed-off-by: Yuyang Du <yuyang.du@intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: bsegall@google.com Cc: dietmar.eggemann@arm.com Cc: matt@codeblueprint.co.uk Cc: morten.rasmussen@arm.com Cc: pjt@google.com Cc: umgwanakikbuti@gmail.com Cc: vincent.guittot@linaro.org Link: http://lkml.kernel.org/r/1486935863-25251-3-git-send-email-yuyang.du@intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
-rw-r--r--kernel/sched/fair.c212
1 files changed, 118 insertions, 94 deletions
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2ac00cfbf29f..76f67b3e34d6 100644
--- 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(u64 val, u64 n)
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)
{
- 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;
+
+ 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];
+ 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) */
+
+ /*
+ * Step 1: decay old *_sum if we crossed period boundaries.
+ */
+ 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);
+ }
+
+ /*
+ * 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;
+ 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, struct sched_avg *sa,
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