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>
Reviewed-by: Tejun Heo <tj@kernel.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
index 827104fb9364cf60df2359e45c3336f61d4568c5..f3cd299fcc41203f9fe73ad16cd70aa77a421ef3 100644 (file)
--- 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