From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752861AbaBMAdU (ORCPT ); Wed, 12 Feb 2014 19:33:20 -0500 Received: from e35.co.us.ibm.com ([32.97.110.153]:60410 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751462AbaBMAdQ (ORCPT ); Wed, 12 Feb 2014 19:33:16 -0500 Date: Wed, 12 Feb 2014 16:33:11 -0800 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Tejun Heo , Kevin Hilman , Lai Jiangshan , Zoran Markovic , linux-kernel@vger.kernel.org, Shaibal Dutta , Dipankar Sarma Subject: Re: [RFC PATCH] rcu: move SRCU grace period work to power efficient workqueue Message-ID: <20140213003311.GJ4250@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1391197986-12774-1-git-send-email-zoran.markovic@linaro.org> <52F8A51F.4090909@cn.fujitsu.com> <20140210184729.GL4250@linux.vnet.ibm.com> <20140212182336.GD5496@localhost.localdomain> <20140212190241.GD4250@linux.vnet.ibm.com> <20140212192354.GC26809@htj.dyndns.org> <20140212195922.GE4250@linux.vnet.ibm.com> <20140212230454.GA14383@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140212230454.GA14383@localhost.localdomain> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14021300-6688-0000-0000-0000066EE9EF Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > Cc: Frederic Weisbecker > > Cc: Tejun Heo > > > > 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 > > 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 Reviewed-by: Tejun Heo Acked-by: Frederic Weisbecker 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