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* [PATCH] sched/fair: remove redundant test_idle_cores for non-smt
@ 2021-03-20 22:14 ` Barry Song
  0 siblings, 0 replies; 13+ messages in thread
From: Barry Song @ 2021-03-20 22:14 UTC (permalink / raw)
  To: vincent.guittot, mingo, peterz, juri.lelli, dietmar.eggemann,
	rostedt, bsegall, mgorman
  Cc: valentin.schneider, aubrey.li, linux-arm-kernel, linux-kernel,
	xuwei5, prime.zeng, guodong.xu, yangyicong, liguozhu, linuxarm,
	Barry Song

update_idle_core() is only done for the case of sched_smt_present.
but test_idle_cores() is done for all machines even those without
smt.
this could contribute to up 8%+ hackbench performance loss on a
machine like kunpeng 920 which has no smt. this patch removes the
redundant test_idle_cores() for non-smt machines.

we run the below hackbench with different -g parameter from 2 to
14, for each different g, we run the command 10 times and get the
average time:
$ numactl -N 0 hackbench -p -T -l 20000 -g $1

hackbench will report the time which is needed to complete a certain
number of messages transmissions between a certain number of tasks,
for example:
$ numactl -N 0 hackbench -p -T -l 20000 -g 10
Running in threaded mode with 10 groups using 40 file descriptors each
(== 400 tasks)
Each sender will pass 20000 messages of 100 bytes

The below is the result of hackbench w/ and w/o this patch:
g=    2      4     6       8      10     12      14
w/o: 1.8151 3.8499 5.5142 7.2491 9.0340 10.7345 12.0929
w/ : 1.8428 3.7436 5.4501 6.9522 8.2882  9.9535 11.3367
                          +4.1%  +8.3%  +7.3%   +6.3%

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
---
 kernel/sched/fair.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2e2ab1e..de42a32 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6038,9 +6038,11 @@ static inline bool test_idle_cores(int cpu, bool def)
 {
 	struct sched_domain_shared *sds;
 
-	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
-	if (sds)
-		return READ_ONCE(sds->has_idle_cores);
+	if (static_branch_likely(&sched_smt_present)) {
+		sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
+		if (sds)
+			return READ_ONCE(sds->has_idle_cores);
+	}
 
 	return def;
 }
-- 
1.8.3.1


^ permalink raw reply related	[flat|nested] 13+ messages in thread

end of thread, other threads:[~2021-03-23 15:09 UTC | newest]

Thread overview: 13+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-03-20 22:14 [PATCH] sched/fair: remove redundant test_idle_cores for non-smt Barry Song
2021-03-20 22:14 ` Barry Song
2021-03-22  4:36 ` Li, Aubrey
2021-03-22  4:36   ` Li, Aubrey
2021-03-22  5:08   ` [Linuxarm] " Song Bao Hua (Barry Song)
2021-03-22  5:08     ` Song Bao Hua (Barry Song)
2021-03-22 10:15 ` Mel Gorman
2021-03-22 10:15   ` Mel Gorman
2021-03-22 11:09 ` Peter Zijlstra
2021-03-22 11:09   ` Peter Zijlstra
2021-03-22 13:11 ` Vincent Guittot
2021-03-22 13:11   ` Vincent Guittot
2021-03-23 15:08 ` [tip: sched/core] sched/fair: Optimize test_idle_cores() for !SMT tip-bot2 for Barry Song

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