linux-kernel - [PATCH -rt] timer: upper bound on loops of __run

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Message-ID: <20141230195228.GA3057@amt.cnet>
Date:	Tue, 30 Dec 2014 17:52:28 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Clark Williams <clark.williams@...il.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org
Subject: [PATCH -rt] timer: upper bound on loops of __run_timers processing


Commit "timers: do not raise softirq unconditionally", allows for timer
wheel processing (__run_timers) to be delayed for long periods of time.

The effect is that 

loops = jiffies - base->timer_jiffies

Can grow to very large values resulting in __run_timers taking hundreds
of milliseconds to execute.

Fix by creating an upper bound on the number of loops to be processed.
This allows a nohz=off kernel to achieve desired latencies.

Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>

diff --git a/kernel/timer.c b/kernel/timer.c
index f59e18c..c128416 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1488,6 +1488,12 @@ void run_local_timers(void)
 	}
 #endif
 
+	if (time_after_eq(jiffies, base->timer_jiffies)) {
+		unsigned long jiffies_delta = jiffies - base->timer_jiffies;
+		if (jiffies_delta > TVR_SIZE)
+			raise_softirq(TIMER_SOFTIRQ);
+	}
+
 	if (!base->active_timers)
 		goto out;
 
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