From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752238Ab3JATIF (ORCPT ); Tue, 1 Oct 2013 15:08:05 -0400 Received: from e28smtp07.in.ibm.com ([122.248.162.7]:33859 "EHLO e28smtp07.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751479Ab3JATIB (ORCPT ); Tue, 1 Oct 2013 15:08:01 -0400 Message-ID: <524B1C94.4020104@linux.vnet.ibm.com> Date: Wed, 02 Oct 2013 00:33:48 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Peter Zijlstra CC: Oleg Nesterov , "Rafael J. Wysocki" , "Paul E. McKenney" , Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Thomas Gleixner , Steven Rostedt , Viresh Kumar Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() References: <20130925175055.GA25914@redhat.com> <20130928144720.GL15690@laptop.programming.kicks-ass.net> <20130928163104.GA23352@redhat.com> <7632387.20FXkuCITr@vostro.rjw.lan> <524B0233.8070203@linux.vnet.ibm.com> <20131001173615.GW3657@laptop.programming.kicks-ass.net> <20131001174508.GA17411@redhat.com> <20131001175640.GQ15690@laptop.programming.kicks-ass.net> In-Reply-To: <20131001175640.GQ15690@laptop.programming.kicks-ass.net> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13100119-8878-0000-0000-0000091113E1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/01/2013 11:26 PM, Peter Zijlstra wrote: > On Tue, Oct 01, 2013 at 07:45:08PM +0200, Oleg Nesterov wrote: >> On 10/01, Peter Zijlstra wrote: >>> >>> On Tue, Oct 01, 2013 at 10:41:15PM +0530, Srivatsa S. Bhat wrote: >>>> However, as Oleg said, its definitely worth considering whether this proposed >>>> change in semantics is going to hurt us in the future. CPU_POST_DEAD has certainly >>>> proved to be very useful in certain challenging situations (commit 1aee40ac9c >>>> explains one such example), so IMHO we should be very careful not to undermine >>>> its utility. >>> >>> Urgh.. crazy things. I've always understood POST_DEAD to mean 'will be >>> called at some time after the unplug' with no further guarantees. And my >>> patch preserves that. >> >> I tend to agree with Srivatsa... Without a strong reason it would be better >> to preserve the current logic: "some time after" should not be after the >> next CPU_DOWN/UP*. But I won't argue too much. > > Nah, I think breaking it is the right thing :-) > >> But note that you do not strictly need this change. Just kill cpuhp_waitcount, >> then we can change cpu_hotplug_begin/end to use xxx_enter/exit we discuss in >> another thread, this should likely "join" all synchronize_sched's. > > That would still be 4k * sync_sched() == terribly long. > >> Or split cpu_hotplug_begin() into 2 helpers which handle FAST -> SLOW and >> SLOW -> BLOCK transitions, then move the first "FAST -> SLOW" handler outside >> of for_each_online_cpu(). > > Right, that's more messy but would work if we cannot teach cpufreq (and > possibly others) to not rely on state you shouldn't rely on anyway. > > I tihnk the only guarnatee POST_DEAD should have is that it should be > called before UP_PREPARE of the same cpu ;-) Nothing more, nothing less. > Conceptually, that hints at a totally per-cpu implementation of CPU hotplug, in which what happens to one CPU doesn't affect the others in the hotplug path.. and yeah, that sounds very tempting! ;-) but I guess that will need to be preceded by a massive rework of many of the existing hotplug callbacks ;-) Regards, Srivatsa S. Bhat