[4.2.y-ckt stable] Patch "vmstat: explicitly schedule per-cpu work on the CPU we need it to run on" 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

    vmstat: explicitly schedule per-cpu work on the CPU we need it to run on

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-ckt5.

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 e34f002ec416e9f4c058a40632f29246abd252ef Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds at linux-foundation.org>
Date: Thu, 15 Oct 2015 13:01:50 -0700
Subject: vmstat: explicitly schedule per-cpu work on the CPU we need it to run
 on

commit 176bed1de5bf977938cad26551969eca8f0883b1 upstream.

The vmstat code uses "schedule_delayed_work_on()" to do the initial
startup of the delayed work on the right CPU, but then once it was
started it would use the non-cpu-specific "schedule_delayed_work()" to
re-schedule it on that CPU.

That just happened to schedule it on the same CPU historically (well, in
almost all situations), but the code _requires_ this work to be per-cpu,
and should say so explicitly rather than depend on the non-cpu-specific
scheduling to schedule on the current CPU.

The timer code is being changed to not be as single-minded in always
running things on the calling CPU.

See also commit 874bbfe600a6 ("workqueue: make sure delayed work run in
local cpu") that for now maintains the local CPU guarantees just in case
there are other broken users that depended on the accidental behavior.

Cc: Christoph Lameter <cl at linux.com>
Cc: Tejun Heo <tj at kernel.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
[ kamal: backport to 3.19-stable: use queue_delayed_work_on() ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/vmstat.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 2139b23..6c9749b 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1366,16 +1366,16 @@ static cpumask_var_t cpu_stat_off;

 static void vmstat_update(struct work_struct *w)
 {
-	if (refresh_cpu_vm_stats(true))
+	if (refresh_cpu_vm_stats(true)) {
 		/*
 		 * Counters were updated so we expect more updates
 		 * to occur in the future. Keep on running the
 		 * update worker thread.
 		 */
-		queue_delayed_work(vmstat_wq,
+		queue_delayed_work_on(smp_processor_id(), vmstat_wq,
 			this_cpu_ptr(&vmstat_work),
 			round_jiffies_relative(sysctl_stat_interval));
-	else {
+	} else {
 		/*
 		 * We did not update any counters so the app may be in
 		 * a mode where it does not cause counter updates.
--
2.7.0