Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751403Ab3JAR46 (ORCPT ); Tue, 1 Oct 2013 13:56:58 -0400 Received: from merlin.infradead.org ([205.233.59.134]:56094 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751079Ab3JAR44 (ORCPT ); Tue, 1 Oct 2013 13:56:56 -0400 Date: Tue, 1 Oct 2013 19:56:40 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: "Srivatsa S. Bhat" , "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() Message-ID: <20131001175640.GQ15690@laptop.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131001174508.GA17411@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1970 Lines: 40 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. -- 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/