From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f51.google.com (mail-pa0-f51.google.com [209.85.220.51]) by kanga.kvack.org (Postfix) with ESMTP id 4E4536B0031 for ; Tue, 1 Oct 2013 10:45:47 -0400 (EDT) Received: by mail-pa0-f51.google.com with SMTP id kp14so7536566pab.24 for ; Tue, 01 Oct 2013 07:45:46 -0700 (PDT) Received: from /spool/local by e9.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 1 Oct 2013 10:45:43 -0400 Received: from b01cxnp23034.gho.pok.ibm.com (b01cxnp23034.gho.pok.ibm.com [9.57.198.29]) by d01dlp03.pok.ibm.com (Postfix) with ESMTP id AB4C4C9004A for ; Tue, 1 Oct 2013 10:45:39 -0400 (EDT) Received: from d03av06.boulder.ibm.com (d03av06.boulder.ibm.com [9.17.195.245]) by b01cxnp23034.gho.pok.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r91EjdKp9699612 for ; Tue, 1 Oct 2013 14:45:39 GMT Received: from d03av06.boulder.ibm.com (loopback [127.0.0.1]) by d03av06.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r91Emh6Z012104 for ; Tue, 1 Oct 2013 08:48:44 -0600 Date: Tue, 1 Oct 2013 07:45:37 -0700 From: "Paul E. McKenney" Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() Message-ID: <20131001144537.GC5790@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <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> <20131001141429.GA32423@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131001141429.GA32423@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Oleg Nesterov Cc: Peter Zijlstra , Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Thomas Gleixner , Steven Rostedt On Tue, Oct 01, 2013 at 04:14:29PM +0200, Oleg Nesterov wrote: > 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(). If you don't have cpuhp_seq, you need some other way to avoid counter overflow. Which might be provided by limited number of tasks, or, on 64-bit systems, 64-bit counters. Thanx, Paul -- 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