Re: [PATCH] sched: Reduce contention in update_cfs_rq_blocked_load

[Jason Low <jason.low2@hp.com>]

On Mon, 2014-08-04 at 13:52 -0700, bsegall@google.com wrote:
> 
> That said, it might be better to remove force_update for this function,
> or make it just reduce the minimum to /64 or something. If the test is
> easy to run it would be good to see what it's like just removing the
> force_update param for this function to see if it's worth worrying
> about or if the zero case catches ~all the perf gain.

Hi Ben,

I removed the force update in __update_cfs_rq_tg_load_contrib and it
helped reduce overhead a lot more. I saw up to a 20x reduction in system
overhead from update_cfs_rq_blocked_load when running some of the AIM7
workloads with this change.

-----
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fea7d33..7a6e18b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2352,8 +2352,7 @@ static inline u64 __synchronize_entity_decay(struct sched_entity *se)
 }
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
-static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
-						 int force_update)
+static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq)
 {
 	struct task_group *tg = cfs_rq->tg;
 	long tg_contrib;
@@ -2361,7 +2360,7 @@ static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
 	tg_contrib = cfs_rq->runnable_load_avg + cfs_rq->blocked_load_avg;
 	tg_contrib -= cfs_rq->tg_load_contrib;
 
-	if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
+	if (abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
 		atomic_long_add(tg_contrib, &tg->load_avg);
 		cfs_rq->tg_load_contrib += tg_contrib;
 	}
@@ -2436,8 +2435,7 @@ static inline void update_rq_runnable_avg(struct rq *rq, int runnable)
 	__update_tg_runnable_avg(&rq->avg, &rq->cfs);
 }
 #else /* CONFIG_FAIR_GROUP_SCHED */
-static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
-						 int force_update) {}
+static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq) {}
 static inline void __update_tg_runnable_avg(struct sched_avg *sa,
 						  struct cfs_rq *cfs_rq) {}
 static inline void __update_group_entity_contrib(struct sched_entity *se) {}
@@ -2537,7 +2535,7 @@ static void update_cfs_rq_blocked_load(struct cfs_rq *cfs_rq, int force_update)
 		cfs_rq->last_decay = now;
 	}
 
-	__update_cfs_rq_tg_load_contrib(cfs_rq, force_update);
+	__update_cfs_rq_tg_load_contrib(cfs_rq);
 }
 
 /* Add the load generated by se into cfs_rq's child load-average */