linux-kernel - [PATCH] sched/fair: Cleanup definition of NOHZ blocked load functions

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Date:   Sun,  2 Jun 2019 17:41:10 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, mingo@...nel.org, vincent.guittot@...aro.org,
        Qian Cai <cai@....pw>
Subject: [PATCH] sched/fair: Cleanup definition of NOHZ blocked load functions

cfs_rq_has_blocked() and others_have_blocked() are only used within
update_blocked_averages(). The !CONFIG_FAIR_GROUP_SCHED version of the
latter calls them within a #define CONFIG_NO_HZ_COMMON block, whereas
the CONFIG_FAIR_GROUP_SCHED one calls them unconditionnally.

As reported by Qian, the above leads to this warning in
!CONFIG_NO_HZ_COMMON configs:

  kernel/sched/fair.c: In function 'update_blocked_averages':
  kernel/sched/fair.c:7750:7: warning: variable 'done' set but not used
  [-Wunused-but-set-variable]

It wouldn't be wrong to keep cfs_rq_has_blocked() and
others_have_blocked() as they are, but since their only current use is
to figure out when we can stop calling update_blocked_averages() on
fully decayed NOHZ idle CPUs, we can give them a new definition for
!CONFIG_NO_HZ_COMMON.

Change the definition of cfs_rq_has_blocked() and
others_have_blocked() for !CONFIG_NO_HZ_COMMON so that the
NOHZ-specific blocks of update_blocked_averages() become no-ops and
the 'done' variable gets optimised out.

No change in functionality intended.

Reported-by: Qian Cai <cai@....pw>
Signed-off-by: Valentin Schneider <valentin.schneider@....com>
---
 kernel/sched/fair.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f35930f5e528..03919a316a03 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7695,6 +7695,7 @@ static void attach_tasks(struct lb_env *env)
 	rq_unlock(env->dst_rq, &rf);
 }
 
+#ifdef CONFIG_NO_HZ_COMMON
 static inline bool cfs_rq_has_blocked(struct cfs_rq *cfs_rq)
 {
 	if (cfs_rq->avg.load_avg)
@@ -7721,6 +7722,10 @@ static inline bool others_have_blocked(struct rq *rq)
 
 	return false;
 }
+#else
+static inline bool cfs_rq_has_blocked(struct cfs_rq *cfs_rq) { return false; }
+static inline bool others_have_blocked(struct rq *rq) { return false; }
+#endif
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 
@@ -7741,6 +7746,18 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
 	return true;
 }
 
+#ifdef CONFIG_NO_HZ_COMMON
+static inline void update_blocked_load_status(struct rq *rq, bool has_blocked)
+{
+	rq->last_blocked_load_update_tick = jiffies;
+
+	if (!has_blocked)
+		rq->has_blocked_load = 0;
+}
+#else
+static inline void update_blocked_load_status(struct rq *rq, bool has_blocked) {}
+#endif
+
 static void update_blocked_averages(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
@@ -7787,11 +7804,7 @@ static void update_blocked_averages(int cpu)
 	if (others_have_blocked(rq))
 		done = false;
 
-#ifdef CONFIG_NO_HZ_COMMON
-	rq->last_blocked_load_update_tick = jiffies;
-	if (done)
-		rq->has_blocked_load = 0;
-#endif
+	update_blocked_load_status(rq, !done);
 	rq_unlock_irqrestore(rq, &rf);
 }
 
-- 
2.20.1