Re: [PATCH v2] sched: Reduce contention in update_cfs_rq_blocked_load

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

On Mon, 2014-09-01 at 14:55 +0200, Peter Zijlstra wrote:
> But yes, unbounded errors here are a problem, sure relaxing the updates
> makes things go fast, they also make things go skew.

Okay. In that case, would you like to take our original patch which
avoids unnecessary updates?

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Subject: [PATCH] sched: Reduce contention in update_cfs_rq_blocked_load

When running workloads on 2+ socket systems, based on perf profiles, the
update_cfs_rq_blocked_load function often shows up as taking up a
noticeable % of run time.

Much of the contention is in __update_cfs_rq_tg_load_contrib when we
update the tg load contribution stats.  However, it turns out that in many
cases, they don't need to be updated and "tg_contrib" is 0.

This patch adds a check in __update_cfs_rq_tg_load_contrib to skip updating
tg load contribution stats when nothing needs to be updated. This reduces the
cacheline contention that would be unnecessary.

Cc: Yuyang Du <yuyang.du@intel.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Chegu Vinod <chegu_vinod@hp.com>
Cc: Scott J Norton <scott.norton@hp.com>
Reviewed-by: Ben Segall <bsegall@google.com>
Reviewed-by: Waiman Long <Waiman.Long@hp.com>
Signed-off-by: Jason Low <jason.low2@hp.com>
---
 kernel/sched/fair.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d3427a8..45e346c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2382,6 +2382,9 @@ 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 (!tg_contrib)
+		return;
+
 	if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
 		atomic_long_add(tg_contrib, &tg->load_avg);
 		cfs_rq->tg_load_contrib += tg_contrib;
-- 
1.7.1