Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752159Ab3JASPZ (ORCPT ); Tue, 1 Oct 2013 14:15:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:21109 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751860Ab3JASPW (ORCPT ); Tue, 1 Oct 2013 14:15:22 -0400 Date: Tue, 1 Oct 2013 20:07:50 +0200 From: Oleg Nesterov To: Peter Zijlstra 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: <20131001180750.GA18261@redhat.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131001175640.GQ15690@laptop.programming.kicks-ass.net> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1852 Lines: 48 On 10/01, Peter Zijlstra wrote: > > On Tue, Oct 01, 2013 at 07:45:08PM +0200, Oleg Nesterov wrote: > > > > 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 :-) I don't really agree but I won't argue ;) > > 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. No? the next xxx_enter() avoids sync_sched() if rcu callback is still pending. Unless __cpufreq_remove_dev_finish() is "too slow" of course. > > 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. Yes, > 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. See above... This makes POST_DEAD really "special" compared to other CPU_* events. And again. Something like a global lock taken by CPU_DOWN_PREPARE and released by POST_DEAD or DOWN_FAILED does not look "too wrong" to me. But I leave this to you and Srivatsa. Oleg. -- 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/