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>

1 files changed, 12 insertions, 1 deletions

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