linux-kernel - Re: [PATCH -next v2] sched/fair: fix -Wunused-but-set-variable warnings

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Date:   Tue, 3 Sep 2019 16:15:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qian Cai <cai@....pw>
Cc:     mingo@...hat.com, bsegall@...gle.com, chiluk+linux@...eed.com,
        pauld@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2] sched/fair: fix -Wunused-but-set-variable
 warnings

On Tue, Sep 03, 2019 at 09:03:26AM -0400, Qian Cai wrote:
> Ingo or Peter, please take a look at this trivial patch. Still see the warning
> in linux-next every day.
> 
> On Tue, 2019-08-20 at 14:40 -0400, Qian Cai wrote:
> > The linux-next commit "sched/fair: Fix low cpu usage with high
> > throttling by removing expiration of cpu-local slices" [1] introduced a
> > few compilation warnings,
> > 
> > kernel/sched/fair.c: In function '__refill_cfs_bandwidth_runtime':
> > kernel/sched/fair.c:4365:6: warning: variable 'now' set but not used
> > [-Wunused-but-set-variable]
> > kernel/sched/fair.c: In function 'start_cfs_bandwidth':
> > kernel/sched/fair.c:4992:6: warning: variable 'overrun' set but not used
> > [-Wunused-but-set-variable]
> > 
> > Also, __refill_cfs_bandwidth_runtime() does no longer update the
> > expiration time, so fix the comments accordingly.
> > 
> > [1] https://lore.kernel.org/lkml/1558121424-2914-1-git-send-email-chiluk+linux
> > @indeed.com/
> > 
> > Signed-off-by: Qian Cai <cai@....pw>

Rewrote the Changelog like so:

---
Subject: sched/fair: Fix -Wunused-but-set-variable warnings
From: Qian Cai <cai@....pw>
Date: Tue, 20 Aug 2019 14:40:55 -0400

Commit de53fd7aedb1 ("sched/fair: Fix low cpu usage with high
throttling by removing expiration of cpu-local slices") introduced a
few compilation warnings:

  kernel/sched/fair.c: In function '__refill_cfs_bandwidth_runtime':
  kernel/sched/fair.c:4365:6: warning: variable 'now' set but not used [-Wunused-but-set-variable]
  kernel/sched/fair.c: In function 'start_cfs_bandwidth':
  kernel/sched/fair.c:4992:6: warning: variable 'overrun' set but not used [-Wunused-but-set-variable]

Also, __refill_cfs_bandwidth_runtime() does no longer update the
expiration time, so fix the comments accordingly.

Fixes: de53fd7aedb1 ("sched/fair: Fix low cpu usage with high throttling by removing expiration of cpu-local slices")
Signed-off-by: Qian Cai <cai@....pw>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Ben Segall <bsegall@...gle.com>
Reviewed-by: Dave Chiluk <chiluk+linux@...eed.com>
Cc: mingo@...hat.com
Cc: pauld@...hat.com
Link: https://lkml.kernel.org/r/1566326455-8038-1-git-send-email-cai@lca.pw
---
 kernel/sched/fair.c |   19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4386,21 +4386,16 @@ static inline u64 sched_cfs_bandwidth_sl
 }
 
 /*
- * Replenish runtime according to assigned quota and update expiration time.
- * We use sched_clock_cpu directly instead of rq->clock to avoid adding
- * additional synchronization around rq->lock.
+ * Replenish runtime according to assigned quota. We use sched_clock_cpu
+ * directly instead of rq->clock to avoid adding additional synchronization
+ * around rq->lock.
  *
  * requires cfs_b->lock
  */
 void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
 {
-	u64 now;
-
-	if (cfs_b->quota == RUNTIME_INF)
-		return;
-
-	now = sched_clock_cpu(smp_processor_id());
-	cfs_b->runtime = cfs_b->quota;
+	if (cfs_b->quota != RUNTIME_INF)
+		cfs_b->runtime = cfs_b->quota;
 }
 
 static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
@@ -5021,15 +5016,13 @@ static void init_cfs_rq_runtime(struct c
 
 void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
 {
-	u64 overrun;
-
 	lockdep_assert_held(&cfs_b->lock);
 
 	if (cfs_b->period_active)
 		return;
 
 	cfs_b->period_active = 1;
-	overrun = hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
+	hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
 	hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
 }