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

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Date:	Wed, 12 Feb 2014 11:59:22 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Kevin Hilman <khilman@...aro.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 02:23:54PM -0500, Tejun Heo wrote:
> Hello,
> 
> On Wed, Feb 12, 2014 at 11:02:41AM -0800, Paul E. McKenney wrote:
> > +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".
> 
> One thing to be careful about is that once published, it becomes part
> of userland visible interface.  Maybe adding some words warning
> against sprinkling WQ_SYSFS willy-nilly is a good idea?

Good point!  How about the following?

							Thanx, Paul

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

Documentation/kernel-per-CPU-kthreads.txt: Workqueue affinity

This commit documents the ability to apply CPU affinity to WQ_SYSFS
workqueues, thus offloading them from the desired worker CPUs.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Tejun Heo <tj@...nel.org>

diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
index 827104fb9364..214da3a47a68 100644
--- a/Documentation/kernel-per-CPU-kthreads.txt
+++ b/Documentation/kernel-per-CPU-kthreads.txt
@@ -162,7 +162,16 @@ 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".  That said, the workqueues
+	maintainer would like to caution people against indiscriminately
+	sprinkling WQ_SYSFS across all the workqueues.  The reason for
+	caution is that it is easy to add WQ_SYSFS, but because sysfs
+	is part of the formal user/kernel API, it can be nearly impossible
+	to remove it, even if its addition was a mistake.
+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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