From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f41.google.com (mail-pb0-f41.google.com [209.85.160.41]) by kanga.kvack.org (Postfix) with ESMTP id DBBA06B0036 for ; Tue, 1 Oct 2013 14:14:49 -0400 (EDT) Received: by mail-pb0-f41.google.com with SMTP id rp2so7524263pbb.0 for ; Tue, 01 Oct 2013 11:14:49 -0700 (PDT) Date: Tue, 1 Oct 2013 20:07:50 +0200 From: Oleg Nesterov 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> Sender: owner-linux-mm@kvack.org List-ID: 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 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org