From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757528Ab3DYKXT (ORCPT ); Thu, 25 Apr 2013 06:23:19 -0400 Received: from mail.skyhub.de ([78.46.96.112]:37922 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755971Ab3DYKXR (ORCPT ); Thu, 25 Apr 2013 06:23:17 -0400 Date: Thu, 25 Apr 2013 12:23:12 +0200 From: Borislav Petkov To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, sbw@mit.edu, Frederic Weisbecker , Steven Rostedt , Arjan van de Ven , Kevin Hilman , Christoph Lameter , Thomas Gleixner , Olivier Baetz Subject: Re: [PATCH documentation 2/2] kthread: Document ways of reducing OS jitter due to per-CPU kthreads Message-ID: <20130425102311.GA32607@pd.tnic> References: <20130416164036.GA27246@linux.vnet.ibm.com> <1366130490-27584-1-git-send-email-paulmck@linux.vnet.ibm.com> <1366130490-27584-2-git-send-email-paulmck@linux.vnet.ibm.com> <20130421193705.GB5807@pd.tnic> <20130423040329.GP3509@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20130423040329.GP3509@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 22, 2013 at 09:03:29PM -0700, Paul E. McKenney wrote: > > > +This document lists per-CPU kthreads in the Linux kernel and presents > > > +options to control OS jitter due to these kthreads. Note that kthreads > > > > s/due to/which can be caused by/ > > Same meaning, but "due to" is probably a bit more arcane. Yeah, "due to" kinda didn't read right in the context, arcane could be one way to put it. > But how about "and presents options to control these kthreads' OS > jitter"? Yep. > > > +that are not per-CPU are not listed here -- to reduce OS jitter from > > > > one too many "that"s: > > > > s/that/which/ > > Fair point, but I can shorten it as follows: > > Note that non-per-CPU kthreads CPU are not listed here -- that second "CPU" is kinda superfluous...? > to reduce OS jitter from non-per-CPU kthreads, bind them to a > "housekeeping" CPU that is dedicated to such work. Yep, reads ok, except "that is" but you've removed it in the final version below. > > > +non-per-CPU kthreads, bind them to a "housekeeping" CPU that is dedicated > > > > s/that/which/ > > Good catch -- I chose s/that is//. Yep. > > > +Name: ehca_comp/%u > > > +Purpose: Periodically process Infiniband-related work. > > > +To reduce corresponding OS jitter, do any of the following: > > > +1. Don't use EHCA Infiniband hardware. This will prevent these > > > > Sounds like this particular hardware is slow and its IRQ handler/softirq > > needs a lot of time. Yes, no? > > > > Can we have a reason why people shouldn't use that hw. > > Because it has per-CPU kthreads that can cause OS jitter. ;-) Yeah, I stumbled over this specific brand of Infiniband hw. It looks like this particular Infiniband driver uses per-CPU kthreads and the others in drivers/infiniband/hw/ don't? I hope this explains my head-scratching moment here... > > This sentence keeps repeating; maybe explain the purpose of this doc in > > the beginning once and drop this sentence in the later sections. > > There are "any of" and "all of" qualifiers. Also, I cannot count on > someone reading the document beginning to end. I would instead expect > many of them to search for the name of the kthread that is bothering > them and read only that part. Ha! Very good point. :-) > > > +2. Build with CONFIG_HOTPLUG_CPU=y. After boot completes, force > > > + the CPU offline, then bring it back online. This forces > > > + recurring timers to migrate elsewhere. If you are concerned > > > > We don't migrate them back to that CPU when we online it again, do we? > > Not unless the CPU it migrated to later is taken offline. Good point, > added words to that effect. Yep, good. > > > + to be de-jittered is marked as an adaptive-ticks CPU using the > > > + "nohz_full=" boot parameter. This reduces the number of > > > + scheduler-clock interrupts that the de-jittered CPU receives, > > > + minimizing its chances of being selected to do load balancing, > > > > I don't think there's a "," if the "which... " part refers to the > > previous "load balancing" and not to the whole sentence. > > Good point -- I can reword to: > > This reduces the number of scheduler-clock interrupts that the > de-jittered CPU receives, minimizing its chances of being selected > to do the load balancing work that runs in SCHED_SOFTIRQ context. Yep. > > > + This further reduces the number of scheduler-clock interrupts > > > + that the de-jittered CPU receives. > > > > s/that/which/ would suit better here IMHO. > > Fair point, but how about this? > > This further reduces the number of scheduler-clock interrupts > received by the de-jittered CPU. Even better. > > > + b. To the extent possible, keep the CPU out of the kernel > > > + when it is non-idle, for example, by avoiding system > > > + calls and by forcing both kernel threads and interrupts > > > + to execute elsewhere. > > > +2. Enable RCU to do its processing remotely via dyntick-idle by > > > + doing all of the following: > > > + a. Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y. > > > + b. Ensure that the CPU goes idle frequently, allowing other > > > > I'm ensuring that by selecting the proper workload which has idle > > breathers? > > Yep! Or, equivalently, by adding enough CPUs so that the workload > has idle breathers. Yeah, this sentence could be in the text, since we're explaining everything! :-) > Thank you for the thorough review and comments! Please see below for > an update. Sure, thank you for writing this up for others to read. Reviewed-by: Borislav Petkov > ------------------------------------------------------------------------ > > REDUCING OS JITTER DUE TO PER-CPU KTHREADS > > This document lists per-CPU kthreads in the Linux kernel and presents > options to control these kthreads' OS jitter. Note that non-per-CPU s /these kthreads'/their/ Sorry, I can't help it :) I promise I won't read too much in the rest so as not to beat it to death again :-) > kthreads CPU are not listed here. To reduce OS jitter from non-per-CPU s/CPU // see above. > kthreads, bind them to a "housekeeping" CPU dedicated to such work. [ … ] Ok, it looks good, ship it. :-) -- Regards/Gruss, Boris. Sent from a fat crate under my desk. Formatting is fine. --