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To:     <fweisbec@gmail.com>, <tglx@linutronix.de>, <mingo@kernel.org>,
        <linux-kernel@vger.kernel.org>, Shiyuan Hu <hushiyuan@huawei.com>,
        Hewenliang <hewenliang4@huawei.com>
From:   Yunfeng Ye <yeyunfeng@huawei.com>
Subject: [PATCH] tick/nohz: Reduce the critical region for jiffies_seq
Message-ID: <ac822c72-673e-73e1-9622-c5f12591b373@huawei.com>
Date:   Wed, 11 Nov 2020 17:11:39 +0800
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When nohz or nohz_full is configured, the concurrency calls of
tick_do_update_jiffies64 increases, and the conflict between
jiffies_lock and jiffies_seq increases, especially in multi-core
scenarios.

However, it is unnecessary to update the jiffies_seq lock multiple
times in a tick period, so the critical region of the jiffies_seq
can be reduced to reduce latency overheads. By the way,
last_jiffies_update is protected by jiffies_lock, so reducing the
jiffies_seq critical area is safe.

Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
---
 kernel/time/tick-sched.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index f0199a4ba1ad..41fb1400439b 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -66,11 +66,11 @@ static void tick_do_update_jiffies64(ktime_t now)

 	/* Reevaluate with jiffies_lock held */
 	raw_spin_lock(&jiffies_lock);
-	write_seqcount_begin(&jiffies_seq);

 	delta = ktime_sub(now, last_jiffies_update);
 	if (delta >= tick_period) {

+		write_seqcount_begin(&jiffies_seq);
 		delta = ktime_sub(delta, tick_period);
 		/* Pairs with the lockless read in this function. */
 		WRITE_ONCE(last_jiffies_update,
@@ -91,12 +91,11 @@ static void tick_do_update_jiffies64(ktime_t now)

 		/* Keep the tick_next_period variable up to date */
 		tick_next_period = ktime_add(last_jiffies_update, tick_period);
-	} else {
 		write_seqcount_end(&jiffies_seq);
+	} else {
 		raw_spin_unlock(&jiffies_lock);
 		return;
 	}
-	write_seqcount_end(&jiffies_seq);
 	raw_spin_unlock(&jiffies_lock);
 	update_wall_time();
 }
-- 
2.18.4