linux-kernel - [PATCH] sched/pelt: fix update_blocked

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Date:   Fri, 31 Aug 2018 15:58:28 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Cc:     Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH] sched/pelt: fix update_blocked_averages() for dl and rt

update_blocked_averages() is called to periodiccally decay the stalled load
of idle CPUs and to sync all loads before running load balance.

When cfs rq is idle, it trigs a load balance during pick_next_task_fair()
in order to potentially pull tasks and to use this newly idle CPU. This
load balance happens whereas prev task from another class has not been put
and its utilization updated yet. This may lead to wrongly account running
time as idle time for rt or dl classes.

Test that no rt or dl task is running when updating their utilization in
update_blocked_averages().

We still update rt and dl utilization instead of simply skipping them to
make sure that all metrics are synced when used during load balance.

Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 309c93f..a1babaf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7262,6 +7262,7 @@ static void update_blocked_averages(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
 	struct cfs_rq *cfs_rq, *pos;
+	const struct sched_class *curr_class = rq->curr->sched_class;
 	struct rq_flags rf;
 	bool done = true;

@@ -7298,8 +7299,8 @@ static void update_blocked_averages(int cpu)
 		if (cfs_rq_has_blocked(cfs_rq))
 			done = false;
 	}
-	update_rt_rq_load_avg(rq_clock_task(rq), rq, 0);
-	update_dl_rq_load_avg(rq_clock_task(rq), rq, 0);
+	update_rt_rq_load_avg(rq_clock_task(rq), rq, curr_class == &rt_sched_class);
+	update_dl_rq_load_avg(rq_clock_task(rq), rq, curr_class == &dl_sched_class);
 	update_irq_load_avg(rq, 0);
 	/* Don't need periodic decay once load/util_avg are null */
 	if (others_have_blocked(rq))
-- 
2.7.4