[PATCH v2 1/2] sched/fair: Check rd->overload value before update

[Shrikanth Hegde]

Overload is an indicator used to inform if there is any rq in the root
domain has 2 or more running tasks. Root domain is a global structure per
cpuset island.

Overload status is updated when adding a task to the runqueue.
It is cleared in update_sd_lb_stats during load balance, if none of the
rq has 2 or more running task. It is used during to newidle
balance to see if its worth doing load balance. If it is set, then
newidle balance will try aggressively to pull a task.

Since commit 630246a06ae2 ("sched/fair: Clean-up update_sg_lb_stats
parameters"), overload is being updated unconditionally. The change in
value of this depends on the workload. On typical workloads, it is
likely that this value doesn't change often. Write causes the
cacheline to be invalidated. This would cause true sharing when the same
variable is accessed in newidle_balance. On large systems this would
cause more CPU bus traffic.

Perf probe stats for reference. Detailed info on perf probe can be found
in the cover letter. With Patch while running hackbench.
1M probe:newidle_balance_L38
139 probe:update_sd_lb_stats_L53
129K probe:add_nr_running_L12
74 probe:add_nr_running_L13
54K probe:update_sd_lb_stats_L50

Perf probes prove that actual change in value is less often. L50 vs L53.
So it would be better to check the value before updating it. CPU bus
traffic reduces with the patch.

Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 02d4d224b436..eeebadd7d9ae 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10621,7 +10621,8 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd

 	if (!env->sd->parent) {
 		/* update overload indicator if we are at root domain */
-		WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD);
+		if (READ_ONCE(env->dst_rq->rd->overload) != (sg_status & SG_OVERLOAD))
+			WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD);

 		/* Update over-utilization (tipping point, U >= 0) indicator */
 		set_rd_overutilized_status(env->dst_rq->rd,
--
2.39.3