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Subject[PATCH v2] sched/fair: Remove setting task's se->runnable_weight during PELT update
Date
A CFS (SCHED_OTHER, SCHED_BATCH or SCHED_IDLE policy) task's
se->runnable_weight must always be in sync with its se->load.weight.

se->runnable_weight is set to se->load.weight when the task is
forked (init_entity_runnable_average()) or reniced (reweight_entity()).

There are two cases in set_load_weight() which since they currently only
set se->load.weight could lead to a situation in which se->load.weight
is different to se->runnable_weight for a CFS task:

(1) A task switches to SCHED_IDLE.

(2) A SCHED_FIFO, SCHED_RR or SCHED_DEADLINE task which has been reniced
(during which only its static priority gets set) switches to
SCHED_OTHER or SCHED_BATCH.

Set se->runnable_weight to se->load.weight in these two cases to prevent
this. This eliminates the need to explicitly set it to se->load.weight
during PELT updates in the CFS scheduler fastpath.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---

Changes v1->v2:
- Rebased on latest tip/sched/core

This patch has been tested w/ appropriate BUG_ON()'s in
__update_load_avg_blocked_se() and __update_load_avg_se() on an Ubuntu
18.04 desktop.

kernel/sched/core.c | 2 ++
kernel/sched/pelt.c | 6 ------
2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index deafa9fe602b..2a08418db3d1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -701,6 +701,7 @@ static void set_load_weight(struct task_struct *p, bool update_load)
if (idle_policy(p->policy)) {
load->weight = scale_load(WEIGHT_IDLEPRIO);
load->inv_weight = WMULT_IDLEPRIO;
+ p->se.runnable_weight = load->weight;
return;
}

@@ -713,6 +714,7 @@ static void set_load_weight(struct task_struct *p, bool update_load)
} else {
load->weight = scale_load(sched_prio_to_weight[prio]);
load->inv_weight = sched_prio_to_wmult[prio];
+ p->se.runnable_weight = load->weight;
}
}

diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
index 35475c0c5419..d0016b16d23a 100644
--- a/kernel/sched/pelt.c
+++ b/kernel/sched/pelt.c
@@ -269,9 +269,6 @@ ___update_load_avg(struct sched_avg *sa, unsigned long load, unsigned long runna

int __update_load_avg_blocked_se(u64 now, int cpu, struct sched_entity *se)
{
- if (entity_is_task(se))
- se->runnable_weight = se->load.weight;
-
if (___update_load_sum(now, cpu, &se->avg, 0, 0, 0)) {
___update_load_avg(&se->avg, se_weight(se), se_runnable(se));
return 1;
@@ -282,9 +279,6 @@ int __update_load_avg_blocked_se(u64 now, int cpu, struct sched_entity *se)

int __update_load_avg_se(u64 now, int cpu, struct cfs_rq *cfs_rq, struct sched_entity *se)
{
- if (entity_is_task(se))
- se->runnable_weight = se->load.weight;
-
if (___update_load_sum(now, cpu, &se->avg, !!se->on_rq, !!se->on_rq,
cfs_rq->curr == se)) {

--
2.11.0
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