sched/fair: Avoid redundant idle_cpu() call in update_sg_lb_stats()

Part of the responsibility of the update_sg_lb_stats() function is to
update the idle_cpus statistical counter in struct sg_lb_stats. This
check is done by calling idle_cpu(). The idle_cpu() function, in
turn, checks a number of fields within the run queue structure such
as rq->curr and rq->nr_running.

With the current layout of the run queue structure, rq->curr and
rq->nr_running are in separate cachelines. The rq->curr variable is
checked first followed by nr_running. As nr_running is also accessed
by update_sg_lb_stats() earlier, it makes no sense to load another
cacheline when nr_running is not 0 as idle_cpu() will always return
false in this case.

This patch eliminates this redundant cacheline load by checking the
cached nr_running before calling idle_cpu().

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1448478580-26467-2-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit a426f99c91d1036767a7819aaaba6bd3191b7f06)
Signed-off-by: Javi Merino <javi.merino@arm.com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e7f9afe29c61a1af8d5b406c9e321897a04635e0..efa516dfd6bc585e7b5bf2a6846ef29ad7e9bde1 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7308,7 +7308,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
                        bool *overload, bool *overutilized)
 {
        unsigned long load;
-       int i;
+       int i, nr_running;
 
        memset(sgs, 0, sizeof(*sgs));
 
@@ -7325,7 +7325,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
                sgs->group_util += cpu_util(i);
                sgs->sum_nr_running += rq->cfs.h_nr_running;
 
-               if (rq->nr_running > 1)
+               nr_running = rq->nr_running;
+               if (nr_running > 1)
                        *overload = true;
 
 #ifdef CONFIG_NUMA_BALANCING
@@ -7333,7 +7334,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
                sgs->nr_preferred_running += rq->nr_preferred_running;
 #endif
                sgs->sum_weighted_load += weighted_cpuload(i);
-               if (idle_cpu(i))
+               /*
+                * No need to call idle_cpu() if nr_running is not 0
+                */
+               if (!nr_running && idle_cpu(i))
                        sgs->idle_cpus++;
 
                if (cpu_overutilized(i)) {