Re: [PATCH v3 10/13] sched/fair: Compute task/cpu utilization at wake-up more correctly

[Morten Rasmussen <morten.rasmussen@arm.com>]

On Mon, Aug 15, 2016 at 04:42:37PM +0100, Morten Rasmussen wrote:
> On Mon, Aug 15, 2016 at 04:23:42PM +0200, Peter Zijlstra wrote:
> > But unlike that function, it doesn't actually use __update_load_avg().
> > Why not?
> 
> Fair question :)
> 
> We currently exploit the fact that the task utilization is _not_ updated
> in wake-up balancing to make sure we don't under-estimate the capacity
> requirements for tasks that have slept for a while. If we update it, we
> loose the non-decayed 'peak' utilization, but I guess we could just
> store it somewhere when we do the wake-up decay.
> 
> I thought there was a better reason when I wrote the patch, but I don't
> recall right now. I will look into it again and see if we can use
> __update_load_avg() to do a proper update instead of doing things twice.

AFAICT, we should be able to synchronize the task utilization to the
previous rq utilization using __update_load_avg() as you suggest. The
patch below is should work as a replacement without any changes to
subsequent patches. It doesn't solve the under-estimation issue, but I
have another patch for that.

---8<---

>From 43226a896fad077c3ab4932f797df17159779d6e Mon Sep 17 00:00:00 2001
From: Morten Rasmussen <morten.rasmussen@arm.com>
Date: Thu, 28 Apr 2016 09:52:35 +0100
Subject: [PATCH] sched/fair: Compute task/cpu utilization at wake-up more
 correctly

At task wake-up load-tracking isn't updated until the task is enqueued.
The task's own view of its utilization contribution may therefore not be
aligned with its contribution to the cfs_rq load-tracking which may have
been updated in the meantime. Basically, the task's own utilization
hasn't yet accounted for the sleep decay, while the cfs_rq may have
(partially). Estimating the cfs_rq utilization in case the task is
migrated at wake-up as task_rq(p)->cfs.avg.util_avg - p->se.avg.util_avg
is therefore incorrect as the two load-tracking signals aren't time
synchronized (different last update).

To solve this problem, this patch synchronizes the task utilization with
its previous rq before the task utilization is used in the wake-up path.
Currently the update/synchronization is done _after_ the task has been
placed by select_task_rq_fair(). The synchronization is done without
having to take the rq lock using the existing mechanism used in
remove_entity_load_avg().

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
---
 kernel/sched/fair.c | 39 +++++++++++++++++++++++++++++++++++----
 1 file changed, 35 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9e217eff3daf..8b6b8f9da28d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3119,13 +3119,25 @@ static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
 #endif
 
 /*
+ * Synchronize entity load avg of dequeued entity without locking
+ * the previous rq.
+ */
+void sync_entity_load_avg(struct sched_entity *se)
+{
+	struct cfs_rq *cfs_rq = cfs_rq_of(se);
+	u64 last_update_time;
+
+	last_update_time = cfs_rq_last_update_time(cfs_rq);
+	__update_load_avg(last_update_time, cpu_of(rq_of(cfs_rq)), &se->avg, 0, 0, NULL);
+}
+
+/*
  * Task first catches up with cfs_rq, and then subtract
  * itself from the cfs_rq (task must be off the queue now).
  */
 void remove_entity_load_avg(struct sched_entity *se)
 {
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
-	u64 last_update_time;
 
 	/*
 	 * tasks cannot exit without having gone through wake_up_new_task() ->
@@ -3137,9 +3149,7 @@ void remove_entity_load_avg(struct sched_entity *se)
 	 * calls this.
 	 */
 
-	last_update_time = cfs_rq_last_update_time(cfs_rq);
-
-	__update_load_avg(last_update_time, cpu_of(rq_of(cfs_rq)), &se->avg, 0, 0, NULL);
+	sync_entity_load_avg(se);
 	atomic_long_add(se->avg.load_avg, &cfs_rq->removed_load_avg);
 	atomic_long_add(se->avg.util_avg, &cfs_rq->removed_util_avg);
 }
@@ -5377,6 +5387,24 @@ static inline int task_util(struct task_struct *p)
 	return p->se.avg.util_avg;
 }
 
+/*
+ * cpu_util_wake: Compute cpu utilization with any contributions from
+ * the waking task p removed.
+ */
+static int cpu_util_wake(int cpu, struct task_struct *p)
+{
+	unsigned long util, capacity;
+
+	/* Task has no contribution or is new */
+	if (cpu != task_cpu(p) || !p->se.avg.last_update_time)
+		return cpu_util(cpu);
+
+	capacity = capacity_orig_of(cpu);
+	util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0);
+
+	return (util >= capacity) ? capacity : util;
+}
+
 static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
 {
 	long min_cap, max_cap;
@@ -5388,6 +5416,9 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
 	if (max_cap - min_cap < max_cap >> 3)
 		return 0;
 
+	/* Bring task utilization in sync with prev_cpu */
+	sync_entity_load_avg(&p->se);
+
 	return min_cap * 1024 < task_util(p) * capacity_margin;
 }
 
-- 
1.9.1