[tip:sched/core] sched/fair: Update and fix the runnable propagation rule

[tip-bot for Vincent Guittot]

Commit-ID:  a4c3c04974d648ee6e1a09ef4131eb32a02ab494
Gitweb:     https://git.kernel.org/tip/a4c3c04974d648ee6e1a09ef4131eb32a02ab494
Author:     Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate: Thu, 16 Nov 2017 15:21:52 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 6 Dec 2017 19:30:50 +0100

sched/fair: Update and fix the runnable propagation rule

Unlike running, the runnable part can't be directly propagated through
the hierarchy when we migrate a task. The main reason is that runnable
time can be shared with other sched_entities that stay on the rq and
this runnable time will also remain on prev cfs_rq and must not be
removed.

Instead, we can estimate what should be the new runnable of the prev
cfs_rq and check that this estimation stay in a possible range. The
prop_runnable_sum is a good estimation when adding runnable_sum but
fails most often when we remove it. Instead, we could use the formula
below instead:

  gcfs_rq's runnable_sum = gcfs_rq->avg.load_sum / gcfs_rq->load.weight

which assumes that tasks are equally runnable which is not true but
easy to compute.

Beside these estimates, we have several simple rules that help us to filter
out wrong ones:

 - ge->avg.runnable_sum <= than LOAD_AVG_MAX
 - ge->avg.runnable_sum >= ge->avg.running_sum (ge->avg.util_sum << LOAD_AVG_MAX)
 - ge->avg.runnable_sum can't increase when we detach a task

The effect of these fixes is better cgroups balancing.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Chris Mason <clm@xxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Yuyang Du <yuyang.du@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1510842112-21028-1-git-send-email-vincent.guittot@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/fair.c | 102 +++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 73 insertions(+), 29 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4037e19..2fe3aa8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3413,9 +3413,9 @@ void set_task_rq_fair(struct sched_entity *se,
  * _IFF_ we look at the pure running and runnable sums. Because they
  * represent the very same entity, just at different points in the hierarchy.
  *
- *
- * Per the above update_tg_cfs_util() is trivial (and still 'wrong') and
- * simply copies the running sum over.
+ * Per the above update_tg_cfs_util() is trivial and simply copies the running
+ * sum over (but still wrong, because the group entity and group rq do not have
+ * their PELT windows aligned).
  *
  * However, update_tg_cfs_runnable() is more complex. So we have:
  *
@@ -3424,11 +3424,11 @@ void set_task_rq_fair(struct sched_entity *se,
  * And since, like util, the runnable part should be directly transferable,
  * the following would _appear_ to be the straight forward approach:
  *
- *   grq->avg.load_avg = grq->load.weight * grq->avg.running_avg	(3)
+ *   grq->avg.load_avg = grq->load.weight * grq->avg.runnable_avg	(3)
  *
  * And per (1) we have:
  *
- *   ge->avg.running_avg == grq->avg.running_avg
+ *   ge->avg.runnable_avg == grq->avg.runnable_avg
  *
  * Which gives:
  *
@@ -3447,27 +3447,28 @@ void set_task_rq_fair(struct sched_entity *se,
  * to (shortly) return to us. This only works by keeping the weights as
  * integral part of the sum. We therefore cannot decompose as per (3).
  *
- * OK, so what then?
+ * Another reason this doesn't work is that runnable isn't a 0-sum entity.
+ * Imagine a rq with 2 tasks that each are runnable 2/3 of the time. Then the
+ * rq itself is runnable anywhere between 2/3 and 1 depending on how the
+ * runnable section of these tasks overlap (or not). If they were to perfectly
+ * align the rq as a whole would be runnable 2/3 of the time. If however we
+ * always have at least 1 runnable task, the rq as a whole is always runnable.
  *
+ * So we'll have to approximate.. :/
  *
- * Another way to look at things is:
+ * Given the constraint:
  *
- *   grq->avg.load_avg = \Sum se->avg.load_avg
+ *   ge->avg.running_sum <= ge->avg.runnable_sum <= LOAD_AVG_MAX
  *
- * Therefore, per (2):
+ * We can construct a rule that adds runnable to a rq by assuming minimal
+ * overlap.
  *
- *   grq->avg.load_avg = \Sum se->load.weight * se->avg.runnable_avg
+ * On removal, we'll assume each task is equally runnable; which yields:
  *
- * And the very thing we're propagating is a change in that sum (someone
- * joined/left). So we can easily know the runnable change, which would be, per
- * (2) the already tracked se->load_avg divided by the corresponding
- * se->weight.
+ *   grq->avg.runnable_sum = grq->avg.load_sum / grq->load.weight
  *
- * Basically (4) but in differential form:
+ * XXX: only do this for the part of runnable > running ?
  *
- *   d(runnable_avg) += se->avg.load_avg / se->load.weight
- *								   (5)
- *   ge->avg.load_avg += ge->load.weight * d(runnable_avg)
  */
 
 static inline void
@@ -3479,6 +3480,14 @@ update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq
 	if (!delta)
 		return;
 
+	/*
+	 * The relation between sum and avg is:
+	 *
+	 *   LOAD_AVG_MAX - 1024 + sa->period_contrib
+	 *
+	 * however, the PELT windows are not aligned between grq and gse.
+	 */
+
 	/* Set new sched_entity's utilization */
 	se->avg.util_avg = gcfs_rq->avg.util_avg;
 	se->avg.util_sum = se->avg.util_avg * LOAD_AVG_MAX;
@@ -3491,33 +3500,68 @@ update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq
 static inline void
 update_tg_cfs_runnable(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq)
 {
-	long runnable_sum = gcfs_rq->prop_runnable_sum;
-	long runnable_load_avg, load_avg;
-	s64 runnable_load_sum, load_sum;
+	long delta_avg, running_sum, runnable_sum = gcfs_rq->prop_runnable_sum;
+	unsigned long runnable_load_avg, load_avg;
+	u64 runnable_load_sum, load_sum = 0;
+	s64 delta_sum;
 
 	if (!runnable_sum)
 		return;
 
 	gcfs_rq->prop_runnable_sum = 0;
 
+	if (runnable_sum >= 0) {
+		/*
+		 * Add runnable; clip at LOAD_AVG_MAX. Reflects that until
+		 * the CPU is saturated running == runnable.
+		 */
+		runnable_sum += se->avg.load_sum;
+		runnable_sum = min(runnable_sum, (long)LOAD_AVG_MAX);
+	} else {
+		/*
+		 * Estimate the new unweighted runnable_sum of the gcfs_rq by
+		 * assuming all tasks are equally runnable.
+		 */
+		if (scale_load_down(gcfs_rq->load.weight)) {
+			load_sum = div_s64(gcfs_rq->avg.load_sum,
+				scale_load_down(gcfs_rq->load.weight));
+		}
+
+		/* But make sure to not inflate se's runnable */
+		runnable_sum = min(se->avg.load_sum, load_sum);
+	}
+
+	/*
+	 * runnable_sum can't be lower than running_sum
+	 * As running sum is scale with cpu capacity wehreas the runnable sum
+	 * is not we rescale running_sum 1st
+	 */
+	running_sum = se->avg.util_sum /
+		arch_scale_cpu_capacity(NULL, cpu_of(rq_of(cfs_rq)));
+	runnable_sum = max(runnable_sum, running_sum);
+
 	load_sum = (s64)se_weight(se) * runnable_sum;
 	load_avg = div_s64(load_sum, LOAD_AVG_MAX);
 
-	add_positive(&se->avg.load_sum, runnable_sum);
-	add_positive(&se->avg.load_avg, load_avg);
+	delta_sum = load_sum - (s64)se_weight(se) * se->avg.load_sum;
+	delta_avg = load_avg - se->avg.load_avg;
 
-	add_positive(&cfs_rq->avg.load_avg, load_avg);
-	add_positive(&cfs_rq->avg.load_sum, load_sum);
+	se->avg.load_sum = runnable_sum;
+	se->avg.load_avg = load_avg;
+	add_positive(&cfs_rq->avg.load_avg, delta_avg);
+	add_positive(&cfs_rq->avg.load_sum, delta_sum);
 
 	runnable_load_sum = (s64)se_runnable(se) * runnable_sum;
 	runnable_load_avg = div_s64(runnable_load_sum, LOAD_AVG_MAX);
+	delta_sum = runnable_load_sum - se_weight(se) * se->avg.runnable_load_sum;
+	delta_avg = runnable_load_avg - se->avg.runnable_load_avg;
 
-	add_positive(&se->avg.runnable_load_sum, runnable_sum);
-	add_positive(&se->avg.runnable_load_avg, runnable_load_avg);
+	se->avg.runnable_load_sum = runnable_sum;
+	se->avg.runnable_load_avg = runnable_load_avg;
 
 	if (se->on_rq) {
-		add_positive(&cfs_rq->avg.runnable_load_avg, runnable_load_avg);
-		add_positive(&cfs_rq->avg.runnable_load_sum, runnable_load_sum);
+		add_positive(&cfs_rq->avg.runnable_load_avg, delta_avg);
+		add_positive(&cfs_rq->avg.runnable_load_sum, delta_sum);
 	}
 }