[4.2.y-ckt stable] Patch "watchdog: don't run proc_watchdog_update if new value is same as old" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

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

    watchdog: don't run proc_watchdog_update if new value is same as old

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt7.

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

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 23607ec8b7a839eceb8b570050caa30ebb9f820a Mon Sep 17 00:00:00 2001
From: Joshua Hunt <johunt at akamai.com>
Date: Thu, 17 Mar 2016 14:17:23 -0700
Subject: watchdog: don't run proc_watchdog_update if new value is same as old

commit a1ee1932aa6bea0bb074f5e3ced112664e4637ed upstream.

While working on a script to restore all sysctl params before a series of
tests I found that writing any value into the
/proc/sys/kernel/{nmi_watchdog,soft_watchdog,watchdog,watchdog_thresh}
causes them to call proc_watchdog_update().

  NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
  NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
  NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
  NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.

There doesn't appear to be a reason for doing this work every time a write
occurs, so only do it when the values change.

Signed-off-by: Josh Hunt <johunt at akamai.com>
Acked-by: Don Zickus <dzickus at redhat.com>
Reviewed-by: Aaron Tomlin <atomlin at redhat.com>
Cc: Ulrich Obergfell <uobergfe at redhat.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/watchdog.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index a6ffa43..8389aaf 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -823,6 +823,9 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
 		 * Update the run state of the lockup detectors.
 		 * Restore 'watchdog_enabled' on failure.
 		 */
+		if (old == new)
+			goto out;
+
 		err = proc_watchdog_update();
 		if (err)
 			watchdog_enabled = old;
@@ -868,7 +871,7 @@ int proc_soft_watchdog(struct ctl_table *table, int write,
 int proc_watchdog_thresh(struct ctl_table *table, int write,
 			 void __user *buffer, size_t *lenp, loff_t *ppos)
 {
-	int err, old;
+	int err, old, new;

 	mutex_lock(&watchdog_proc_mutex);

@@ -882,6 +885,10 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
 	 * Update the sample period.
 	 * Restore 'watchdog_thresh' on failure.
 	 */
+	new = ACCESS_ONCE(watchdog_thresh);
+	if (old == new)
+		goto out;
+
 	set_sample_period();
 	err = proc_watchdog_update();
 	if (err)
--
2.7.4