Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933385Ab2FEBHP (ORCPT ); Mon, 4 Jun 2012 21:07:15 -0400 Received: from ozlabs.org ([203.10.76.45]:47551 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757538Ab2FEBHJ (ORCPT ); Mon, 4 Jun 2012 21:07:09 -0400 From: Rusty Russell To: Peter Zijlstra , Thomas Gleixner Cc: Fenghua Yu , Ingo Molnar , H Peter Anvin , Suresh B Siddha , Tony Luck , Asit K Mallick , Arjan Dan De Ven , linux-kernel , x86 , linux-pm , "Srivatsa S. Bhat" Subject: Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or nmi In-Reply-To: <1338842001.28282.135.camel@twins> References: <1338833876-29721-1-git-send-email-fenghua.yu@intel.com> <1338842001.28282.135.camel@twins> User-Agent: Notmuch/0.12 (http://notmuchmail.org) Emacs/23.3.1 (i686-pc-linux-gnu) Date: Tue, 05 Jun 2012 10:10:33 +0930 Message-ID: <87zk8iioam.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2584 Lines: 59 On Mon, 04 Jun 2012 22:33:21 +0200, Peter Zijlstra wrote: > On Mon, 2012-06-04 at 22:11 +0200, Thomas Gleixner wrote: > > > I understand what you are trying to do, though I completely disagree > > with the solution. > > > > The main problem of the current hotplug code is that it is an all or > > nothing approach. You have to tear down the whole thing completely > > instead of just taking it out of the usable set of cpus. > > > > I'm working on a proper state machine driven online/offline sequence, > > where you can put the cpu into an intermediate state which avoids > > bringing it down completely. This is enough to get the full > > powersaving benefits w/o having to go through all the synchronization > > states of a full online/offline. That will shorten the onlining time > > of an previously offlined cpu to almost nothing. > > > > I really want to avoid adding more bandaids to the hotplug code before > > we have sorted out the existing horror. > > Its far worse.. you shouldn't _ever_ care about hotplug latency unless > you've got absolutely braindead hardware. We all now ARM has been > particularly creative here, but is Intel now trying to trump ARM at > stupid? I disagree. Deactivating a cpu for power saving is halfway to hotplug anyway. I'd rather unify the two cases, where we can specify how dead a CPU should be, than have individual archs and boards do random hacks. It also gives us a great excuse to audit and neaten various of the hotplug cpu callbacks; most of the ones I've looked at have been racy :( The ones which simply want to keep per-cpu stats can be given a nice helper with two simple callbacks: one to empty stats for a going-away cpu, and (maybe) one to restore them. The per-cpu kthreads should no longer get torn down and recreated, and doing it via a separate notifier function is ugly and error-prone. My plan is a "bool kthread_cpu_going(void)" and then a "void kthread_cpu_can_go(void)", so kthreads can do: if (kthread_cpu_going()) { /* Do any cleanup we need. */ ... /* This returns when CPU comes back. */ kthread_cpu_can_go(); } Yeah, we should probably have the kthread exit inside kthread_cpu_can_go() if they stop the kthread, but that's a detail. Cheers, Rusty. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/