linux-kernel - [PATCH] vmstat_update: ensure work remains on the same core

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Date:	Fri, 16 Oct 2015 15:51:33 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Michal Hocko <mhocko@...e.cz>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Shaohua Li <shli@...com>, linux-mm <linux-mm@...ck.org>,
	Tejun Heo <tj@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Chris Metcalf <cmetcalf@...hip.com>
Subject: [PATCH] vmstat_update: ensure work remains on the same core

By using schedule_delayed_work(), we are preferring the local
core for the work, but not requiring it.  In my task
isolation experiments, I saw a nohz_full core's vmstat_update
end up running on a housekeeping core, and when the two works
ran back-to-back, we triggered the VM_BUG_ON() at the
end of the function.

Switch to using schedule_delayed_work_on(smp_processor_id(), ...).

Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>
---
This change that I made a few days ago in my local tree is
particularly amusing given that the thread I am appending this
email to ("workqueue fixes for v4.3-rc5") also fixes the symptoms
of the bug I saw, but I wasn't aware of it until just now.  And it
took a while for me to track it down!  I think this is probably a
"belt and suspenders" kind of issue where it makes sense to fix it
on both sides of the API, however.

 mm/vmstat.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index cf7d324f16e2..5c6bd7e5db07 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1369,7 +1369,8 @@ static void vmstat_update(struct work_struct *w)
 		 * to occur in the future. Keep on running the
 		 * update worker thread.
 		 */
-		schedule_delayed_work(this_cpu_ptr(&vmstat_work),
+		schedule_delayed_work_on(smp_processor_id(),
+			this_cpu_ptr(&vmstat_work),
 			round_jiffies_relative(sysctl_stat_interval));
 	else {
 		/*
-- 
2.1.2

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