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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Wed, Feb 12, 2014 at 11:59:22AM -0800, Paul E. McKenney wrote:
> 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@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Tejun Heo <tj@kernel.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

Acked-by: Frederic Weisbecker <fweisbec@gmail.com>

I just suggest we append a small explanation about what WQ_SYSFS is about.
Like:

+2.
+       The workqueues that want to be visible on the sysfs hierarchy
+	set the WQ_SYSFS flag.
+  	For those who have this flag set, you can use the
+	/sys/devices/virtual/workqueue/*/cpumask sysfs files
+	to force the 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