linux-kernel - Re: [RFC PATCH] rcu: move SRCU grace period work to power efficient workqueue

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Date:	Wed, 12 Feb 2014 11:02:41 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Kevin Hilman <khilman@...aro.org>, Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Zoran Markovic <zoran.markovic@...aro.org>,
	linux-kernel@...r.kernel.org,
	Shaibal Dutta <shaibal.dutta@...adcom.com>,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [RFC PATCH] rcu: move SRCU grace period work to power efficient
 workqueue

On Wed, Feb 12, 2014 at 07:23:38PM +0100, Frederic Weisbecker wrote:
> On Mon, Feb 10, 2014 at 10:47:29AM -0800, Paul E. McKenney wrote:
> > On Mon, Feb 10, 2014 at 06:08:31PM +0800, Lai Jiangshan wrote:
> > > Acked-by: Lai Jiangshan <laijs@...fujitsu.com>
> > 
> > Thank you all, queued for 3.15.
> > 
> > We should also have some facility for moving the SRCU workqueues to
> > housekeeping/timekeeping kthreads in the NO_HZ_FULL case.  Or does
> > this patch already have that effect?
> 
> Kevin Hilman and me plan to try to bring a new Kconfig option that could let
> us control the unbound workqueues affinity through sysfs.

Please CC me or feel free to update Documentation/kernel-per-CPU-kthreads.txt
as part of this upcoming series.

> The feature actually exist currently but is only enabled for workqueues that
> have WQ_SYSFS. Writeback and raid5 are the only current users.
> 
> See for example: /sys/devices/virtual/workqueue/writeback/cpumask

Ah, news to me!  I have queued the following patch, seem reasonable?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
index 827104fb9364..09f28841ee3e 100644
--- a/Documentation/kernel-per-CPU-kthreads.txt
+++ b/Documentation/kernel-per-CPU-kthreads.txt
@@ -162,7 +162,11 @@ Purpose: Execute workqueue requests
 To reduce its OS jitter, do any of the following:
 1.	Run your workload at a real-time priority, which will allow
 	preempting the kworker daemons.
-2.	Do any of the following needed to avoid jitter that your
+2.	Use the /sys/devices/virtual/workqueue/*/cpumask sysfs files
+	to force the WQ_SYSFS workqueues to run on the specified set
+	of CPUs.  The set of WQ_SYSFS workqueues can be displayed using
+	"ls sys/devices/virtual/workqueue".
+3.	Do any of the following needed to avoid jitter that your
 	application cannot tolerate:
 	a.	Build your kernel with CONFIG_SLUB=y rather than
 		CONFIG_SLAB=y, thus avoiding the slab allocator's periodic

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