From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: question about rcuc/X tasks Date: Thu, 15 Dec 2016 15:34:30 -0800 Message-ID: <20161215233430.GF3924@linux.vnet.ibm.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> <585317DF.7080502@windriver.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Steven Rostedt , Daniel Bristot de Oliveira , linux-rt-users@vger.kernel.org To: Chris Friesen Return-path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:46591 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754171AbcLOXeg (ORCPT ); Thu, 15 Dec 2016 18:34:36 -0500 Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uBFNXkcg048078 for ; Thu, 15 Dec 2016 18:34:31 -0500 Received: from e32.co.us.ibm.com (e32.co.us.ibm.com [32.97.110.150]) by mx0a-001b2d01.pphosted.com with ESMTP id 27bwsmgs50-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 15 Dec 2016 18:34:30 -0500 Received: from localhost by e32.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 15 Dec 2016 16:34:29 -0700 Content-Disposition: inline In-Reply-To: <585317DF.7080502@windriver.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Thu, Dec 15, 2016 at 04:23:27PM -0600, Chris Friesen wrote: > 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? Interesting. I need to look into this a bit. I would not expect that rcuc/X kthreads corresponding to NOCB CPUs to ever wake up. (They are created by a per-CPU facility that creates a kthread per CPU no matter what.) Thanx, Paul