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


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