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

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Date:	Thu, 13 Feb 2014 15:05:25 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Tejun Heo <tj@...nel.org>, 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 04:33:11PM -0800, Paul E. McKenney wrote:
> 
> Fair point!  I wordsmithed it into the following.  Seem reasonable?
> 
> 							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>
> Reviewed-by: Tejun Heo <tj@...nel.org>
> Acked-by: Frederic Weisbecker <fweisbec@...il.com>
> 
> diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
> index 827104fb9364..f3cd299fcc41 100644
> --- a/Documentation/kernel-per-CPU-kthreads.txt
> +++ b/Documentation/kernel-per-CPU-kthreads.txt
> @@ -162,7 +162,18 @@ 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.	A given workqueue can be made visible in the sysfs filesystem
> +	by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
> +	Such a workqueue can be confined to a given subset of the
> +	CPUs using the /sys/devices/virtual/workqueue/*/cpumask sysfs
> +	files.	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
> 

Perfect!!
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