linux-kernel - [PATCH 05/12] timer: Add comments about calc

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Message-Id: <20200717140551.29076-6-frederic@kernel.org>
Date:   Fri, 17 Jul 2020 16:05:44 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: [PATCH 05/12] timer: Add comments about calc_index() ceiling work

calc_index() adds 1 unit of the level granularity to the expiry passed
in parameter to ensure that the timer doesn't expire too early. Add a
comment to explain that and the resulting layout in the wheel.

Most-of-the-patch-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Juri Lelli <juri.lelli@...hat.com>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc: Juri Lelli <juri.lelli@...hat.com>
---
 kernel/time/timer.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 2af08a169564..af1c08b0b168 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -156,7 +156,8 @@ EXPORT_SYMBOL(jiffies_64);
 
 /*
  * The time start value for each level to select the bucket at enqueue
- * time.
+ * time. We start from the last possible delta of the previous level
+ * so that we can later add an extra LVL_GRAN(n) to n (see calc_index()).
  */
 #define LVL_START(n)	((LVL_SIZE - 1) << (((n) - 1) * LVL_CLK_SHIFT))
 
@@ -490,6 +491,15 @@ static inline void timer_set_idx(struct timer_list *timer, unsigned int idx)
 static inline unsigned calc_index(unsigned long expires, unsigned lvl,
 				  unsigned long *bucket_expiry)
 {
+
+	/*
+	 * The timer wheel has to guarantee that a timer does not fire
+	 * early. Early expiry can happen due to:
+	 * - Timer is armed at the edge of a tick
+	 * - Truncation of the expiry time in the outer wheel levels
+	 *
+	 * Round up with level granularity to prevent this.
+	 */
 	expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
 	*bucket_expiry = expires << LVL_SHIFT(lvl);
 	return LVL_OFFS(lvl) + (expires & LVL_MASK);
-- 
2.26.2