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Date: Wed, 24 Oct 2012 02:43:46 -0700
From: tip-bot for Paul Turner <pjt@...gle.com>
To: linux-tip-commits@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, bsegall@...gle.com, hpa@...or.com,
mingo@...nel.org, a.p.zijlstra@...llo.nl, pjt@...gle.com,
tglx@...utronix.de
Subject: [tip:sched/core] sched:
Track the runnable average on a per-task entity basis
Commit-ID: 9d85f21c94f7f7a84d0ba686c58aa6d9da58fdbb
Gitweb: http://git.kernel.org/tip/9d85f21c94f7f7a84d0ba686c58aa6d9da58fdbb
Author: Paul Turner <pjt@...gle.com>
AuthorDate: Thu, 4 Oct 2012 13:18:29 +0200
Committer: Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 24 Oct 2012 10:27:18 +0200
sched: Track the runnable average on a per-task entity basis
Instead of tracking averaging the load parented by a cfs_rq, we can track
entity load directly. With the load for a given cfs_rq then being the sum
of its children.
To do this we represent the historical contribution to runnable average
within each trailing 1024us of execution as the coefficients of a
geometric series.
We can express this for a given task t as:
runnable_sum(t) = \Sum u_i * y^i, runnable_avg_period(t) = \Sum 1024 * y^i
load(t) = weight_t * runnable_sum(t) / runnable_avg_period(t)
Where: u_i is the usage in the last i`th 1024us period (approximately 1ms)
~ms and y is chosen such that y^k = 1/2. We currently choose k to be 32 which
roughly translates to about a sched period.
Signed-off-by: Paul Turner <pjt@...gle.com>
Reviewed-by: Ben Segall <bsegall@...gle.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Link: http://lkml.kernel.org/r/20120823141506.372695337@google.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
include/linux/sched.h | 13 +++++
kernel/sched/core.c | 5 ++
kernel/sched/debug.c | 4 ++
kernel/sched/fair.c | 129 +++++++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 151 insertions(+), 0 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0dd42a0..418fc6d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1095,6 +1095,16 @@ struct load_weight {
unsigned long weight, inv_weight;
};
+struct sched_avg {
+ /*
+ * These sums represent an infinite geometric series and so are bound
+ * above by 1024/(1-y). Thus we only need a u32 to store them for for all
+ * choices of y < 1-2^(-32)*1024.
+ */
+ u32 runnable_avg_sum, runnable_avg_period;
+ u64 last_runnable_update;
+};
+
#ifdef CONFIG_SCHEDSTATS
struct sched_statistics {
u64 wait_start;
@@ -1155,6 +1165,9 @@ struct sched_entity {
/* rq "owned" by this entity/group: */
struct cfs_rq *my_q;
#endif
+#ifdef CONFIG_SMP
+ struct sched_avg avg;
+#endif
};
struct sched_rt_entity {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2d8927f..fd9d085 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1524,6 +1524,11 @@ static void __sched_fork(struct task_struct *p)
p->se.vruntime = 0;
INIT_LIST_HEAD(&p->se.group_node);
+#ifdef CONFIG_SMP
+ p->se.avg.runnable_avg_period = 0;
+ p->se.avg.runnable_avg_sum = 0;
+#endif
+
#ifdef CONFIG_SCHEDSTATS
memset(&p->se.statistics, 0, sizeof(p->se.statistics));
#endif
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 6f79596..61f7097 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -85,6 +85,10 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
P(se->statistics.wait_count);
#endif
P(se->load.weight);
+#ifdef CONFIG_SMP
+ P(se->avg.runnable_avg_sum);
+ P(se->avg.runnable_avg_period);
+#endif
#undef PN
#undef P
}
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6b800a1..16d67f9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -971,6 +971,126 @@ static inline void update_entity_shares_tick(struct cfs_rq *cfs_rq)
}
#endif /* CONFIG_FAIR_GROUP_SCHED */
+#ifdef CONFIG_SMP
+/*
+ * Approximate:
+ * val * y^n, where y^32 ~= 0.5 (~1 scheduling period)
+ */
+static __always_inline u64 decay_load(u64 val, u64 n)
+{
+ for (; n && val; n--) {
+ val *= 4008;
+ val >>= 12;
+ }
+
+ return val;
+}
+
+/*
+ * 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
+ * occurred N-ms ago p_N, with p_0 corresponding to the current period, e.g.
+ *
+ * [<- 1024us ->|<- 1024us ->|<- 1024us ->| ...
+ * p0 p1 p2
+ * (now) (~1ms ago) (~2ms ago)
+ *
+ * Let u_i denote the fraction of p_i that the entity was runnable.
+ *
+ * We then designate the fractions u_i as our co-efficients, yielding the
+ * following representation of historical load:
+ * u_0 + u_1*y + u_2*y^2 + u_3*y^3 + ...
+ *
+ * We choose y based on the with of a reasonably scheduling period, fixing:
+ * y^32 = 0.5
+ *
+ * This means that the contribution to load ~32ms ago (u_32) will be weighted
+ * approximately half as much as the contribution to load within the last ms
+ * (u_0).
+ *
+ * When a period "rolls over" and we have new u_0`, multiplying the previous
+ * sum again by y is sufficient to update:
+ * load_avg = u_0` + y*(u_0 + u_1*y + u_2*y^2 + ... )
+ * = u_0 + u_1*y + u_2*y^2 + ... [re-labeling u_i --> u_{i+1}]
+ */
+static __always_inline int __update_entity_runnable_avg(u64 now,
+ struct sched_avg *sa,
+ int runnable)
+{
+ u64 delta;
+ int delta_w, decayed = 0;
+
+ delta = now - sa->last_runnable_update;
+ /*
+ * This should only happen when time goes backwards, which it
+ * unfortunately does during sched clock init when we swap over to TSC.
+ */
+ if ((s64)delta < 0) {
+ sa->last_runnable_update = now;
+ return 0;
+ }
+
+ /*
+ * Use 1024ns as the unit of measurement since it's a reasonable
+ * approximation of 1us and fast to compute.
+ */
+ delta >>= 10;
+ if (!delta)
+ return 0;
+ sa->last_runnable_update = now;
+
+ /* delta_w is the amount already accumulated against our next period */
+ delta_w = sa->runnable_avg_period % 1024;
+ if (delta + delta_w >= 1024) {
+ /* period roll-over */
+ decayed = 1;
+
+ /*
+ * 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;
+ BUG_ON(delta_w > delta);
+ do {
+ if (runnable)
+ sa->runnable_avg_sum += delta_w;
+ sa->runnable_avg_period += delta_w;
+
+ /*
+ * Remainder of delta initiates a new period, roll over
+ * the previous.
+ */
+ sa->runnable_avg_sum =
+ decay_load(sa->runnable_avg_sum, 1);
+ sa->runnable_avg_period =
+ decay_load(sa->runnable_avg_period, 1);
+
+ delta -= delta_w;
+ /* New period is empty */
+ delta_w = 1024;
+ } while (delta >= 1024);
+ }
+
+ /* Remainder of delta accrued against u_0` */
+ if (runnable)
+ sa->runnable_avg_sum += delta;
+ sa->runnable_avg_period += delta;
+
+ return decayed;
+}
+
+/* Update a sched_entity's runnable average */
+static inline void update_entity_load_avg(struct sched_entity *se)
+{
+ __update_entity_runnable_avg(rq_of(cfs_rq_of(se))->clock_task, &se->avg,
+ se->on_rq);
+}
+#else
+static inline void update_entity_load_avg(struct sched_entity *se) {}
+#endif
+
static void enqueue_sleeper(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
#ifdef CONFIG_SCHEDSTATS
@@ -1097,6 +1217,7 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
*/
update_curr(cfs_rq);
update_cfs_load(cfs_rq, 0);
+ update_entity_load_avg(se);
account_entity_enqueue(cfs_rq, se);
update_cfs_shares(cfs_rq);
@@ -1171,6 +1292,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
* Update run-time statistics of the 'current'.
*/
update_curr(cfs_rq);
+ update_entity_load_avg(se);
update_stats_dequeue(cfs_rq, se);
if (flags & DEQUEUE_SLEEP) {
@@ -1340,6 +1462,8 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
update_stats_wait_start(cfs_rq, prev);
/* Put 'current' back into the tree. */
__enqueue_entity(cfs_rq, prev);
+ /* in !on_rq case, update occurred at dequeue */
+ update_entity_load_avg(prev);
}
cfs_rq->curr = NULL;
}
@@ -1353,6 +1477,11 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
update_curr(cfs_rq);
/*
+ * Ensure that runnable average is periodically updated.
+ */
+ update_entity_load_avg(curr);
+
+ /*
* Update share accounting for long-running entities.
*/
update_entity_shares_tick(cfs_rq);
--
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