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

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Date:   Mon, 3 Jun 2019 11:38:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        vincent.guittot@...aro.org, Qian Cai <cai@....pw>
Subject: Re: [PATCH] sched/fair: Cleanup definition of NOHZ blocked load
 functions

On Sun, Jun 02, 2019 at 05:41:10PM +0100, Valentin Schneider wrote:
> 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>

I'm thinking the below can go on top to further clean up?

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7722,9 +7722,18 @@ static inline bool others_have_blocked(s
 
 	return false;
 }
+
+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 bool cfs_rq_has_blocked(struct cfs_rq *cfs_rq) { return false; }
 static inline bool others_have_blocked(struct rq *rq) { return false; }
+static inline void update_blocked_load_status(struct rq *rq, bool has_blocked) {}
 #endif
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
@@ -7746,18 +7755,6 @@ static inline bool cfs_rq_is_decayed(str
 	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);
@@ -7870,11 +7867,7 @@ static inline void update_blocked_averag
 	update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
 	update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
 	update_irq_load_avg(rq, 0);
-#ifdef CONFIG_NO_HZ_COMMON
-	rq->last_blocked_load_update_tick = jiffies;
-	if (!cfs_rq_has_blocked(cfs_rq) && !others_have_blocked(rq))
-		rq->has_blocked_load = 0;
-#endif
+	update_blocked_load_status(rq, cfs_rq_has_blocked(cfs_rq) || others_have_blocked(rq));
 	rq_unlock_irqrestore(rq, &rf);
 }