sched/fair: Apply more PELT fixes
One additional 'rule' for using update_cfs_rq_load_avg() is that one
should call update_tg_load_avg() if it returns true.
Add a bunch of comments to hopefully clarify some of the rules:
o You need to update cfs_rq _before_ any entity attach/detach,
this is important, because while for mathmatical consisency this
isn't strictly needed, it is required for the physical
interpretation of the model, you attach/detach _now_.
o When you modify the cfs_rq avg, you have to then call
update_tg_load_avg() in order to propagate changes upwards.
o (Fair) entities are always attached, switched_{to,from}_fair()
deal with !fair. This directly follows from the definition of the
cfs_rq averages, namely that they are a direct sum of all
(runnable or blocked) entities on that rq.
It is the second rule that this patch enforces, but it adds comments
pertaining to all of them.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0c21a12..781788d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -692,6 +692,7 @@
static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq);
static int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq);
+static void update_tg_load_avg(struct cfs_rq *cfs_rq, int force);
static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se);
/*
@@ -725,6 +726,7 @@
struct sched_avg *sa = &se->avg;
long cap = (long)(SCHED_CAPACITY_SCALE - cfs_rq->avg.util_avg) / 2;
u64 now = cfs_rq_clock_task(cfs_rq);
+ int tg_update;
if (cap > 0) {
if (cfs_rq->avg.util_avg != 0) {
@@ -757,8 +759,10 @@
}
}
- update_cfs_rq_load_avg(now, cfs_rq, false);
+ tg_update = update_cfs_rq_load_avg(now, cfs_rq, false);
attach_entity_load_avg(cfs_rq, se);
+ if (tg_update)
+ update_tg_load_avg(cfs_rq, false);
}
#else /* !CONFIG_SMP */
@@ -768,6 +772,9 @@
void post_init_entity_util_avg(struct sched_entity *se)
{
}
+static void update_tg_load_avg(struct cfs_rq *cfs_rq, int force)
+{
+}
#endif /* CONFIG_SMP */
/*
@@ -2912,7 +2919,23 @@
WRITE_ONCE(*ptr, res); \
} while (0)
-/* Group cfs_rq's load_avg is used for task_h_load and update_cfs_share */
+/**
+ * update_cfs_rq_load_avg - update the cfs_rq's load/util averages
+ * @now: current time, as per cfs_rq_clock_task()
+ * @cfs_rq: cfs_rq to update
+ * @update_freq: should we call cfs_rq_util_change() or will the call do so
+ *
+ * The cfs_rq avg is the direct sum of all its entities (blocked and runnable)
+ * avg. The immediate corollary is that all (fair) tasks must be attached, see
+ * post_init_entity_util_avg().
+ *
+ * cfs_rq->avg is used for task_h_load() and update_cfs_share() for example.
+ *
+ * Returns true if the load decayed or we removed utilization. It is expected
+ * that one calls update_tg_load_avg() on this condition, but after you've
+ * modified the cfs_rq avg (attach/detach), such that we propagate the new
+ * avg up.
+ */
static inline int
update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq)
{
@@ -2967,6 +2990,14 @@
update_tg_load_avg(cfs_rq, 0);
}
+/**
+ * attach_entity_load_avg - attach this entity to its cfs_rq load avg
+ * @cfs_rq: cfs_rq to attach to
+ * @se: sched_entity to attach
+ *
+ * Must call update_cfs_rq_load_avg() before this, since we rely on
+ * cfs_rq->avg.last_update_time being current.
+ */
static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
if (!sched_feat(ATTACH_AGE_LOAD))
@@ -2998,6 +3029,14 @@
cfs_rq_util_change(cfs_rq);
}
+/**
+ * detach_entity_load_avg - detach this entity from its cfs_rq load avg
+ * @cfs_rq: cfs_rq to detach from
+ * @se: sched_entity to detach
+ *
+ * Must call update_cfs_rq_load_avg() before this, since we rely on
+ * cfs_rq->avg.last_update_time being current.
+ */
static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
__update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq_of(cfs_rq)),
@@ -8404,6 +8443,7 @@
struct sched_entity *se = &p->se;
struct cfs_rq *cfs_rq = cfs_rq_of(se);
u64 now = cfs_rq_clock_task(cfs_rq);
+ int tg_update;
if (!vruntime_normalized(p)) {
/*
@@ -8415,8 +8455,10 @@
}
/* Catch up with the cfs_rq and remove our load when we leave */
- update_cfs_rq_load_avg(now, cfs_rq, false);
+ tg_update = update_cfs_rq_load_avg(now, cfs_rq, false);
detach_entity_load_avg(cfs_rq, se);
+ if (tg_update)
+ update_tg_load_avg(cfs_rq, false);
}
static void attach_task_cfs_rq(struct task_struct *p)
@@ -8424,6 +8466,7 @@
struct sched_entity *se = &p->se;
struct cfs_rq *cfs_rq = cfs_rq_of(se);
u64 now = cfs_rq_clock_task(cfs_rq);
+ int tg_update;
#ifdef CONFIG_FAIR_GROUP_SCHED
/*
@@ -8434,8 +8477,10 @@
#endif
/* Synchronize task with its cfs_rq */
- update_cfs_rq_load_avg(now, cfs_rq, false);
+ tg_update = update_cfs_rq_load_avg(now, cfs_rq, false);
attach_entity_load_avg(cfs_rq, se);
+ if (tg_update)
+ update_tg_load_avg(cfs_rq, false);
if (!vruntime_normalized(p))
se->vruntime += cfs_rq->min_vruntime;
