[3.11.y.z extended stable] Patch "hrtimer: Prevent all reprogramming if hang detected" has been added to staging queue

Luis Henriques luis.henriques at canonical.com
Wed May 7 11:09:05 UTC 2014


This is a note to let you know that I have just added a patch titled

    hrtimer: Prevent all reprogramming if hang detected

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 04a8755054dd36b72e5bb59137285f085a7affdc Mon Sep 17 00:00:00 2001
From: Stuart Hayes <stuart.w.hayes at gmail.com>
Date: Tue, 29 Apr 2014 17:55:02 -0500
Subject: hrtimer: Prevent all reprogramming if hang detected

commit 6c6c0d5a1c949d2e084706f9e5fb1fccc175b265 upstream.

If the last hrtimer interrupt detected a hang it sets hang_detected=1
and programs the clock event device with a delay to let the system
make progress.

If hang_detected == 1, we prevent reprogramming of the clock event
device in hrtimer_reprogram() but not in hrtimer_force_reprogram().

This can lead to the following situation:

hrtimer_interrupt()
   hang_detected = 1;
   program ce device to Xms from now (hang delay)

We have two timers pending:
   T1 expires 50ms from now
   T2 expires 5s from now

Now T1 gets canceled, which causes hrtimer_force_reprogram() to be
invoked, which in turn programs the clock event device to T2 (5
seconds from now).

Any hrtimer_start after that will not reprogram the hardware due to
hang_detected still being set. So we effectivly block all timers until
the T2 event fires and cleans up the hang situation.

Add a check for hang_detected to hrtimer_force_reprogram() which
prevents the reprogramming of the hang delay in the hardware
timer. The subsequent hrtimer_interrupt will resolve all outstanding
issues.

[ tglx: Rewrote subject and changelog and fixed up the comment in
  	hrtimer_force_reprogram() ]

Signed-off-by: Stuart Hayes <stuart.w.hayes at gmail.com>
Link: http://lkml.kernel.org/r/53602DC6.2060101@gmail.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/hrtimer.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 383319b..0d6df7b 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -581,6 +581,23 @@ hrtimer_force_reprogram(struct hrtimer_cpu_base *cpu_base, int skip_equal)

 	cpu_base->expires_next.tv64 = expires_next.tv64;

+	/*
+	 * If a hang was detected in the last timer interrupt then we
+	 * leave the hang delay active in the hardware. We want the
+	 * system to make progress. That also prevents the following
+	 * scenario:
+	 * T1 expires 50ms from now
+	 * T2 expires 5s from now
+	 *
+	 * T1 is removed, so this code is called and would reprogram
+	 * the hardware to 5s from now. Any hrtimer_start after that
+	 * will not reprogram the hardware due to hang_detected being
+	 * set. So we'd effectivly block all timers until the T2 event
+	 * fires.
+	 */
+	if (cpu_base->hang_detected)
+		return;
+
 	if (cpu_base->expires_next.tv64 != KTIME_MAX)
 		tick_program_event(cpu_base->expires_next, 1);
 }
--
1.9.1





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