From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Friesen Subject: Re: question about rcuc/X tasks Date: Thu, 15 Dec 2016 16:23:27 -0600 Message-ID: <585317DF.7080502@windriver.com> References: <584F27B8.2090406@windriver.com> <518a5f66-76d0-e356-b08b-bde2a7a17bb2@bristot.me> <20161215090714.0b62cc03@gandalf.local.home> <5852B4B8.1090600@windriver.com> <20161215190405.GZ3924@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Cc: Steven Rostedt , Daniel Bristot de Oliveira , To: Return-path: Received: from mail1.windriver.com ([147.11.146.13]:63014 "EHLO mail1.windriver.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753929AbcLOWXl (ORCPT ); Thu, 15 Dec 2016 17:23:41 -0500 In-Reply-To: <20161215190405.GZ3924@linux.vnet.ibm.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On 12/15/2016 01:04 PM, Paul E. McKenney wrote: > On Thu, Dec 15, 2016 at 09:20:24AM -0600, Chris Friesen wrote: >> On a related note, I found an old email from Paul suggesting that >> the various rcuc/X threads could be affined to the management CPUs >> to free up the "realtime" cores, but when I try that it doesn't let >> me change affinity. Was that disallowed for technical reasons? >> (It's also possible it's something local, in which case I need to go >> digging.) > > The rcuo/X kthreads can be affined, but the rcuc/X kthreads must run on > the corresponding CPU for correctness reasons -- they communicate with > RCU core using protocols that are only single-CPU-safe. But if you are > running NO_HZ_FULL, these kthreads should never run unless your user > threads are doing syscalls. > > So, are they actually running in your setup? Yes, but I wasn't setting nohz_full. With "rcu_nocb_poll isolcpus=1-15 rcu_nocbs=1-15 nohz_full=1-15" I'm not seeing the rcuc/X kthreads running. So in the non-nohz_full case, what are they waking up to do? Something timer-related? Thanks, Chris