linux-kernel - [tip:timers/core] timer: Remove FIFO "guarantee"

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Date:	Fri, 19 Jun 2015 06:22:44 -0700
From:	tip-bot for Thomas Gleixner <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	edumazet@...gle.com, paulmck@...ux.vnet.ibm.com,
	john.stultz@...aro.org, linux-kernel@...r.kernel.org,
	linux@...izon.com, hpa@...or.com, mingo@...nel.org,
	fweisbec@...il.com, viresh.kumar@...aro.org, tglx@...utronix.de,
	joonwoop@...eaurora.org, wenbo.wang@...blaze.com,
	peterz@...radead.org
Subject: [tip:timers/core] timer: Remove FIFO "guarantee"

Commit-ID:  1bd04bf6f68d65f5422b2b85c495d65d49587a54
Gitweb:     http://git.kernel.org/tip/1bd04bf6f68d65f5422b2b85c495d65d49587a54
Author:     Thomas Gleixner <tglx@...utronix.de>
AuthorDate: Tue, 26 May 2015 22:50:26 +0000
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 19 Jun 2015 15:18:27 +0200

timer: Remove FIFO "guarantee"

The FIFO guarantee is only there if two timers are queued into the
same bucket at the same jiffie on the same cpu:

 - The slack value depends on the delta between expiry and enqueue
   time, so the resulting expiry time can be different for timers
   which are queued in different jiffies.

 - Timers which are queued into the secondary array end up after a
   later queued timer which was queued into the primary array due to
   cascading.

 - Timers can end up on different cpus due to the NOHZ target moving
   around. Obviously there is no guarantee of expiry ordering between
   cpus.

So anything which relies on FIFO behaviour of the timer wheel is
broken already.

This is a preparatory patch for converting the timer wheel to hlist
which reduces the memory foot print of the wheel by 50%.

It's a seperate patch so any (unlikely to happen) regression caused by
this can be identified clearly.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: John Stultz <john.stultz@...aro.org>
Cc: Joonwoo Park <joonwoop@...eaurora.org>
Cc: Wenbo Wang <wenbo.wang@...blaze.com>
Cc: George Spelvin <linux@...izon.com>
Link: http://lkml.kernel.org/r/20150526224511.757520403@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/timer.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index d5e0179..e212df2 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -389,10 +389,8 @@ __internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 		i = (expires >> (TVR_BITS + 3 * TVN_BITS)) & TVN_MASK;
 		vec = base->tv5.vec + i;
 	}
-	/*
-	 * Timers are FIFO:
-	 */
-	list_add_tail(&timer->entry, vec);
+
+	list_add(&timer->entry, vec);
 }
 
 static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
--
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