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Subject[PATCH 04/16] sched: maintain the load contribution of blocked entities
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We are currently maintaining:
runnable_load(cfs_rq) = \Sum task_load(t)

For all running children t of cfs_rq. While this can be naturally updated for
tasks in a runnable state (as they are scheduled); this does not account for
the load contributed by blocked task entities.

This can be solved by introducing a separate accounting for blocked load:
blocked_load(cfs_rq) = \Sum runnable(b) * weight(b)

Obviously we do not want to iterate over all blocked entities to account for
their decay, we instead observe that:
runnable_load(t) = \Sum p_i*y^i

and that to account for an additional idle period we only need to compute:
y*runnable_load(t).

This means that we can compute all blocked entities at once by evaluating:
blocked_load(cfs_rq)` = y * blocked_load(cfs_rq)

Finally we maintain a decay counter so that when a sleeping entity re-awakens
we can determine how much of its load should be removed from the blocked sum.

Signed-off-by: Paul Turner <pjt@google.com>
Signed-off-by: Ben Segall <bsegall@google.com>
---
include/linux/sched.h | 1
kernel/sched/core.c | 3 +
kernel/sched/debug.c | 3 +
kernel/sched/fair.c | 130 ++++++++++++++++++++++++++++++++++++++++++++-----
kernel/sched/sched.h | 4 +-
5 files changed, 126 insertions(+), 15 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0c54ce0..842c4df 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1139,6 +1139,7 @@ struct load_weight {
struct sched_avg {
u32 runnable_avg_sum, runnable_avg_period;
u64 last_runnable_update;
+ s64 decay_count;
unsigned long load_avg_contrib;
};

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9bb7d28..aeb8e56 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1713,6 +1713,9 @@ static void __sched_fork(struct task_struct *p)
p->se.vruntime = 0;
INIT_LIST_HEAD(&p->se.group_node);

+#if defined(CONFIG_FAIR_GROUP_SCHED) && defined(CONFIG_SMP)
+ p->se.avg.decay_count = 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 aeb74e3..2aa60cf 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -95,6 +95,7 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
P(se->avg.runnable_avg_sum);
P(se->avg.runnable_avg_period);
P(se->avg.load_avg_contrib);
+ P(se->avg.decay_count);
#endif
#undef PN
#undef P
@@ -230,6 +231,8 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
atomic_read(&cfs_rq->tg->load_weight));
SEQ_printf(m, " .%-30s: %lld\n", "runnable_load_avg",
cfs_rq->runnable_load_avg);
+ SEQ_printf(m, " .%-30s: %lld\n", "blocked_load_avg",
+ cfs_rq->blocked_load_avg);
#endif

print_cfs_group_stats(m, cpu, cfs_rq->tg);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8229766..6200d20 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1085,6 +1085,20 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
return decayed;
}

+/* Synchronize an entity's decay with its parentin cfs_rq.*/
+static inline void __synchronize_entity_decay(struct sched_entity *se)
+{
+ struct cfs_rq *cfs_rq = cfs_rq_of(se);
+ u64 decays = atomic64_read(&cfs_rq->decay_counter);
+
+ decays -= se->avg.decay_count;
+ if (!decays)
+ return;
+
+ se->avg.load_avg_contrib = decay_load(se->avg.load_avg_contrib, decays);
+ se->avg.decay_count += decays;
+}
+
/* Compute the current contribution to load_avg by se, return any delta */
static long __update_entity_load_avg_contrib(struct sched_entity *se)
{
@@ -1100,8 +1114,18 @@ static long __update_entity_load_avg_contrib(struct sched_entity *se)
return se->avg.load_avg_contrib - old_contrib;
}

+static inline void subtract_blocked_load_contrib(struct cfs_rq *cfs_rq,
+ long load_contrib)
+{
+ if (likely(load_contrib < cfs_rq->blocked_load_avg))
+ cfs_rq->blocked_load_avg -= load_contrib;
+ else
+ cfs_rq->blocked_load_avg = 0;
+}
+
/* Update a sched_entity's runnable average */
-static inline void update_entity_load_avg(struct sched_entity *se)
+static inline void update_entity_load_avg(struct sched_entity *se,
+ int update_cfs_rq)
{
struct cfs_rq *cfs_rq = cfs_rq_of(se);
long contrib_delta;
@@ -1111,8 +1135,34 @@ static inline void update_entity_load_avg(struct sched_entity *se)
return;

contrib_delta = __update_entity_load_avg_contrib(se);
+
+ if (!update_cfs_rq)
+ return;
+
if (se->on_rq)
cfs_rq->runnable_load_avg += contrib_delta;
+ else
+ subtract_blocked_load_contrib(cfs_rq, -contrib_delta);
+}
+
+/*
+ * Decay the load contributed by all blocked children and account this so that
+ * they their contribution may appropriately discounted when they wake up.
+ */
+static void update_cfs_rq_blocked_load(struct cfs_rq *cfs_rq)
+{
+ u64 now = rq_of(cfs_rq)->clock_task >> 20;
+ u64 decays;
+
+ decays = now - cfs_rq->last_decay;
+ if (!decays)
+ return;
+
+ cfs_rq->blocked_load_avg = decay_load(cfs_rq->blocked_load_avg,
+ decays);
+ atomic64_add(decays, &cfs_rq->decay_counter);
+
+ cfs_rq->last_decay = now;
}

static inline void update_rq_runnable_avg(struct rq *rq, int runnable)
@@ -1122,26 +1172,56 @@ static inline void update_rq_runnable_avg(struct rq *rq, int runnable)

/* Add the load generated by se into cfs_rq's child load-average */
static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
- struct sched_entity *se)
-{
- update_entity_load_avg(se);
+ struct sched_entity *se,
+ int wakeup)
+{
+ /* we track migrations using entity decay_count == 0 */
+ if (unlikely(!se->avg.decay_count)) {
+ se->avg.last_runnable_update = rq_of(cfs_rq)->clock_task;
+ se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter);
+ wakeup = 0;
+ } else {
+ __synchronize_entity_decay(se);
+ }
+
+ if (wakeup)
+ subtract_blocked_load_contrib(cfs_rq, se->avg.load_avg_contrib);
+
+ update_entity_load_avg(se, 0);
cfs_rq->runnable_load_avg += se->avg.load_avg_contrib;
+ update_cfs_rq_blocked_load(cfs_rq);
}

-/* Remove se's load from this cfs_rq child load-average */
+/*
+ * Remove se's load from this cfs_rq child load-average, if the entity is
+ * transitioning to a blocked state we track its projected decay using
+ * blocked_load_avg.
+ */
static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq,
- struct sched_entity *se)
+ struct sched_entity *se,
+ int sleep)
{
- update_entity_load_avg(se);
+ update_entity_load_avg(se, 1);
+
cfs_rq->runnable_load_avg -= se->avg.load_avg_contrib;
+ if (sleep) {
+ cfs_rq->blocked_load_avg += se->avg.load_avg_contrib;
+ se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter);
+ } else {
+ se->avg.decay_count = 0;
+ }
}
#else
-static inline void update_entity_load_avg(struct sched_entity *se) {}
+static inline void update_entity_load_avg(struct sched_entity *se,
+ int update_cfs_rq) {}
static inline void update_rq_runnable_avg(struct rq *rq, int runnable) {}
static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
- struct sched_entity *se) {}
+ struct sched_entity *se,
+ int wakeup) {}
static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq,
- struct sched_entity *se) {}
+ struct sched_entity *se,
+ int sleep) {}
+static inline void update_cfs_rq_blocked_load(struct cfs_rq *cfs_rq) {}
#endif

static void enqueue_sleeper(struct cfs_rq *cfs_rq, struct sched_entity *se)
@@ -1270,7 +1350,7 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
*/
update_curr(cfs_rq);
update_cfs_load(cfs_rq, 0);
- enqueue_entity_load_avg(cfs_rq, se);
+ enqueue_entity_load_avg(cfs_rq, se, flags & ENQUEUE_WAKEUP);
account_entity_enqueue(cfs_rq, se);
update_cfs_shares(cfs_rq);

@@ -1345,7 +1425,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
* Update run-time statistics of the 'current'.
*/
update_curr(cfs_rq);
- dequeue_entity_load_avg(cfs_rq, se);
+ dequeue_entity_load_avg(cfs_rq, se, flags & DEQUEUE_SLEEP);

update_stats_dequeue(cfs_rq, se);
if (flags & DEQUEUE_SLEEP) {
@@ -1516,7 +1596,7 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *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);
+ update_entity_load_avg(prev, 1);
}
cfs_rq->curr = NULL;
}
@@ -1532,7 +1612,8 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
/*
* Ensure that runnable average is periodically updated.
*/
- update_entity_load_avg(curr);
+ update_entity_load_avg(curr, 1);
+ update_cfs_rq_blocked_load(cfs_rq);

/*
* Update share accounting for long-running entities.
@@ -2391,6 +2472,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)

update_cfs_load(cfs_rq, 0);
update_cfs_shares(cfs_rq);
+ update_entity_load_avg(se, 1);
}

if (!se) {
@@ -2452,6 +2534,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)

update_cfs_load(cfs_rq, 0);
update_cfs_shares(cfs_rq);
+ update_entity_load_avg(se, 1);
}

if (!se) {
@@ -3557,6 +3640,7 @@ static int update_shares_cpu(struct task_group *tg, int cpu)

update_rq_clock(rq);
update_cfs_load(cfs_rq, 1);
+ update_cfs_rq_blocked_load(cfs_rq);

/*
* We need to update shares after updating tg->load_weight in
@@ -5379,6 +5463,21 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
place_entity(cfs_rq, se, 0);
se->vruntime -= cfs_rq->min_vruntime;
}
+
+#if defined(CONFIG_FAIR_GROUP_SCHED) && defined(CONFIG_SMP)
+ /*
+ * Remove our load from contribution when we leave sched_fair
+ * and ensure we don't carry in an old decay_count if we
+ * switch back.
+ */
+ if (p->se.avg.decay_count) {
+ struct cfs_rq *cfs_rq = cfs_rq_of(&p->se);
+ __synchronize_entity_decay(&p->se);
+ subtract_blocked_load_contrib(cfs_rq,
+ p->se.avg.load_avg_contrib);
+ p->se.avg.decay_count = 0;
+ }
+#endif
}

/*
@@ -5425,6 +5524,9 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
#ifndef CONFIG_64BIT
cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
#endif
+#if defined(CONFIG_FAIR_GROUP_SCHED) && defined(CONFIG_SMP)
+ atomic64_set(&cfs_rq->decay_counter, 1);
+#endif
}

#ifdef CONFIG_FAIR_GROUP_SCHED
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 26cc36f..a96adf1 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -229,7 +229,9 @@ struct cfs_rq {
* This allows for the description of both thread and group usage (in
* the FAIR_GROUP_SCHED case).
*/
- u64 runnable_load_avg;
+ u64 runnable_load_avg, blocked_load_avg;
+ atomic64_t decay_counter;
+ u64 last_decay;
#endif
#ifdef CONFIG_FAIR_GROUP_SCHED
struct rq *rq; /* cpu runqueue to which this cfs_rq is attached */



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