On Thu, Feb 13, 2014 at 12:04:57AM +0100, Frederic Weisbecker wrote:
> 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 <paul...@linux.vnet.ibm.com>
> > Cc: Frederic Weisbecker <fweis...@gmail.com>
> > Cc: Tejun Heo <t...@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 <fweis...@gmail.com>
>
> I just suggest we append a small explanation about what WQ_SYSFS is about.
> Like:
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 <paul...@linux.vnet.ibm.com>
Reviewed-by: Tejun Heo <t...@kernel.org>
Acked-by: Frederic Weisbecker <fweis...@gmail.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
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