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From:   Barry Song <song.bao.hua@hisilicon.com>
To:     <vincent.guittot@linaro.org>, <mingo@redhat.com>,
        <peterz@infradead.org>, <juri.lelli@redhat.com>,
        <dietmar.eggemann@arm.com>, <rostedt@goodmis.org>,
        <bsegall@google.com>, <mgorman@suse.de>
CC:     <valentin.schneider@arm.com>, <aubrey.li@linux.intel.com>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>, <xuwei5@huawei.com>,
        <prime.zeng@hisilicon.com>, <guodong.xu@linaro.org>,
        <yangyicong@huawei.com>, <liguozhu@hisilicon.com>,
        <linuxarm@openeuler.org>, Barry Song <song.bao.hua@hisilicon.com>
Subject: [PATCH] sched/fair: remove redundant test_idle_cores for non-smt
Date:   Sun, 21 Mar 2021 11:14:32 +1300
Message-ID: <20210320221432.924-1-song.bao.hua@hisilicon.com>
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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