From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f179.google.com (mail-pd0-f179.google.com [209.85.192.179]) by kanga.kvack.org (Postfix) with ESMTP id 7BF8D6B0032 for ; Tue, 1 Oct 2013 10:21:34 -0400 (EDT) Received: by mail-pd0-f179.google.com with SMTP id v10so7351148pde.10 for ; Tue, 01 Oct 2013 07:21:34 -0700 (PDT) Date: Tue, 1 Oct 2013 16:14:29 +0200 From: Oleg Nesterov Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() Message-ID: <20131001141429.GA32423@redhat.com> References: <20130925174307.GA3220@laptop.programming.kicks-ass.net> <20130925175055.GA25914@redhat.com> <20130925184015.GC3657@laptop.programming.kicks-ass.net> <20130925212200.GA7959@linux.vnet.ibm.com> <20130926111042.GS3081@twins.programming.kicks-ass.net> <20130926165840.GA863@redhat.com> <20130926175016.GI3657@laptop.programming.kicks-ass.net> <20130927181532.GA8401@redhat.com> <20130927204116.GJ15690@laptop.programming.kicks-ass.net> <20131001035604.GW19582@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131001035604.GW19582@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: "Paul E. McKenney" Cc: Peter Zijlstra , Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Thomas Gleixner , Steven Rostedt On 09/30, Paul E. McKenney wrote: > > On Fri, Sep 27, 2013 at 10:41:16PM +0200, Peter Zijlstra wrote: > > On Fri, Sep 27, 2013 at 08:15:32PM +0200, Oleg Nesterov wrote: > > > On 09/26, Peter Zijlstra wrote: > > [ . . . ] > > > > > +static bool cpuhp_readers_active_check(void) > > > > { > > > > + unsigned int seq = per_cpu_sum(cpuhp_seq); > > > > + > > > > + smp_mb(); /* B matches A */ > > > > + > > > > + /* > > > > + * In other words, if we see __get_online_cpus() cpuhp_seq increment, > > > > + * we are guaranteed to also see its __cpuhp_refcount increment. > > > > + */ > > > > > > > > + if (per_cpu_sum(__cpuhp_refcount) != 0) > > > > + return false; > > > > > > > > + smp_mb(); /* D matches C */ > > > > > > It seems that both barries could be smp_rmb() ? I am not sure the comments > > > from srcu_readers_active_idx_check() can explain mb(), note that > > > __srcu_read_lock() always succeeds unlike get_cpus_online(). > > > > I see what you mean; cpuhp_readers_active_check() is all purely reads; > > there are no writes to order. > > > > Paul; is there any argument for the MB here as opposed to RMB; and if > > not should we change both these and SRCU? > > Given that these memory barriers execute only on the semi-slow path, > why add the complexity of moving from smp_mb() to either smp_rmb() > or smp_wmb()? Straight smp_mb() is easier to reason about and more > robust against future changes. But otoh this looks misleading, and the comments add more confusion. But please note another email, it seems to me we can simply kill cpuhp_seq and all the barriers in cpuhp_readers_active_check(). 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