[PATCH] sched/fair: remove redundant test_idle_cores for non-smt

[Peter Zijlstra]

On Sun, Mar 21, 2021 at 11:14:32AM +1300, Barry Song wrote:
> 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%
> 

The patch looks obvious, but the Changelog and Subject needed a lot of
help.

I've changed it like so:

---
Subject: sched/fair: Optimize test_idle_cores() for !SMT
From: Barry Song <song.bao.hua at hisilicon.com>
Date: Sun, 21 Mar 2021 11:14:32 +1300

From: Barry Song <song.bao.hua at hisilicon.com>

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 can 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 !SMT machines.

Hackbench is ran with -g {2..14}, for each g it is ran 10 times to get
an average.

  $ numactl -N 0 hackbench -p -T -l 20000 -g $1

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 at hisilicon.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Acked-by: Mel Gorman <mgorman at suse.de>
Link: https://lkml.kernel.org/r/20210320221432.924-1-song.bao.hua@hisilicon.com
---
 kernel/sched/fair.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6038,9 +6038,11 @@ static inline bool test_idle_cores(int c
 {
 	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;
 }