[PATCH v2 1/3] sched/fair: Avoid redundant idle_cpu() call in update_sg_lb_stats()

Waiman Long
2015-12-02T18:42:09Z

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>
---
kernel/sched/fair.c | 10 +++++++---
1 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f04fda8..8f1eccc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6302,7 +6302,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
bool *overload)
{
unsigned long load;
- int i;
+ int i, nr_running;

memset(sgs, 0, sizeof(*sgs));

@@ -6319,7 +6319,8 @@ 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
@@ -6327,7 +6328,10 @@ 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++;
}

--
1.7.1