[tip: timers/core] tick/sched: Reduce seqcount held scope in tick_do_update_jiffies64()

[tip-bot2 for Yunfeng Ye]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     94ad2e3cedb82af034f6d97c58022f162b669f9b
Gitweb:        https://git.kernel.org/tip/94ad2e3cedb82af034f6d97c58022f162b669f9b
Author:        Yunfeng Ye <yeyunfeng@xxxxxxxxxx>
AuthorDate:    Tue, 17 Nov 2020 14:19:46 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 19 Nov 2020 10:48:29 +01:00

tick/sched: Reduce seqcount held scope in tick_do_update_jiffies64()

If jiffies are up to date already (caller lost the race against another
CPU) there is no point to change the sequence count. Doing that just forces
other CPUs into the seqcount retry loop in tick_nohz_next_event() for
nothing.

Just bail out early.

[ tglx: Rewrote most of it ]

Signed-off-by: Yunfeng Ye <yeyunfeng@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20201117132006.462195901@xxxxxxxxxxxxx

---
 kernel/time/tick-sched.c | 47 ++++++++++++++++++---------------------
 1 file changed, 22 insertions(+), 25 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b4b6abc..ca9191c 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -86,38 +86,35 @@ static void tick_do_update_jiffies64(ktime_t now)
 
 	/* Reevaluate with jiffies_lock held */
 	raw_spin_lock(&jiffies_lock);
+	if (ktime_before(now, tick_next_period)) {
+		raw_spin_unlock(&jiffies_lock);
+		return;
+	}
+
 	write_seqcount_begin(&jiffies_seq);
 
-	delta = ktime_sub(now, last_jiffies_update);
-	if (delta >= tick_period) {
+	last_jiffies_update = ktime_add(last_jiffies_update, tick_period);
 
-		delta = ktime_sub(delta, tick_period);
-		last_jiffies_update = ktime_add(last_jiffies_update,
-						tick_period);
+	delta = ktime_sub(now, tick_next_period);
+	if (unlikely(delta >= tick_period)) {
+		/* Slow path for long idle sleep times */
+		s64 incr = ktime_to_ns(tick_period);
 
-		/* Slow path for long timeouts */
-		if (unlikely(delta >= tick_period)) {
-			s64 incr = ktime_to_ns(tick_period);
+		ticks = ktime_divns(delta, incr);
 
-			ticks = ktime_divns(delta, incr);
+		last_jiffies_update = ktime_add_ns(last_jiffies_update,
+						   incr * ticks);
+	}
 
-			last_jiffies_update = ktime_add_ns(last_jiffies_update,
-							   incr * ticks);
-		}
-		do_timer(++ticks);
+	do_timer(++ticks);
+
+	/*
+	 * Keep the tick_next_period variable up to date.  WRITE_ONCE()
+	 * pairs with the READ_ONCE() in the lockless quick check above.
+	 */
+	WRITE_ONCE(tick_next_period,
+		   ktime_add(last_jiffies_update, tick_period));
 
-		/*
-		 * Keep the tick_next_period variable up to date.
-		 * WRITE_ONCE() pairs with the READ_ONCE() in the lockless
-		 * quick check above.
-		 */
-		WRITE_ONCE(tick_next_period,
-			   ktime_add(last_jiffies_update, tick_period));
-	} else {
-		write_seqcount_end(&jiffies_seq);
-		raw_spin_unlock(&jiffies_lock);
-		return;
-	}
 	write_seqcount_end(&jiffies_seq);
 	raw_spin_unlock(&jiffies_lock);
 	update_wall_time();