From: "tip-bot2 for Frederic Weisbecker" <tip-bot2@linutronix.de>
To: linux-tip-commits@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>,
Frederic Weisbecker <frederic@kernel.org>,
Juri Lelli <juri.lelli@redhat.com>, x86 <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: [tip: timers/core] timers: Add comments about calc_index() ceiling work
Date: Fri, 17 Jul 2020 20:00:19 -0000 [thread overview]
Message-ID: <159501601930.4006.7549756767804002018.tip-bot2@tip-bot2> (raw)
In-Reply-To: <20200717140551.29076-6-frederic@kernel.org>
The following commit has been merged into the timers/core branch of tip:
Commit-ID: 4468897211628865ee2392acb5ad281f74176f63
Gitweb: https://git.kernel.org/tip/4468897211628865ee2392acb5ad281f74176f63
Author: Frederic Weisbecker <frederic@kernel.org>
AuthorDate: Fri, 17 Jul 2020 16:05:44 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 17 Jul 2020 21:55:22 +02:00
timers: 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.
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
---
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 2af08a1..af1c08b 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);
next prev parent reply other threads:[~2020-07-17 20:00 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-17 14:05 [PATCH 00/11] timer: Reduce timers softirq v3 Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 01/12] timer: Fix wheel index calculation on last level Frederic Weisbecker
2020-07-17 19:49 ` [tip: timers/urgent] " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 02/12] timer: Preserve higher bits of expiration on index calculation Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] timers: " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 03/12] timers: Use only bucket expiry for base->next_expiry value Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] " tip-bot2 for Anna-Maria Behnsen
2020-07-17 14:05 ` [PATCH 04/12] timer: Move trigger_dyntick_cpu() to enqueue_timer() Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] timers: " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 05/12] timer: Add comments about calc_index() ceiling work Frederic Weisbecker
2020-07-17 20:00 ` tip-bot2 for Frederic Weisbecker [this message]
2020-07-17 14:05 ` [PATCH 06/12] timer: Optimize _next_timer_interrupt() level iteration Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] timers: " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 07/12] timers: Always keep track of next expiry Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 08/12] timer: Reuse next expiry cache after nohz exit Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] timers: " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 09/12] timer: Expand clk forward logic beyond nohz Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] timers: " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 10/12] timer: Spare timer softirq until next expiry Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] timers: " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 11/12] timer: Remove must_forward_clk Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] timers: " tip-bot2 for Frederic Weisbecker
2020-07-17 14:05 ` [PATCH 12/12] timer: Lower base clock forwarding threshold Frederic Weisbecker
2020-07-17 20:00 ` [tip: timers/core] timers: " tip-bot2 for Frederic Weisbecker
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