From 4468897211628865ee2392acb5ad281f74176f63 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@kernel.org>
Date: Fri, 17 Jul 2020 16:05:44 +0200
Subject: [PATCH] timers: Add comments about calc_index() ceiling work
Git-commit: 4468897211628865ee2392acb5ad281f74176f63
Patch-mainline: v5.9-rc1
References: bsc#1179365
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.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200717140551.29076-6-frederic@kernel.org
Signed-off-by: Frederic Weisbecker <fweisbecker@suse.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);
--
2.25.1