From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Sat, 12 May 2007 11:06:24 -0700 From: Andrew Morton Subject: Re: [PATCH 1/2] scalable rw_mutex Message-Id: <20070512110624.9ac3aa44.akpm@linux-foundation.org> In-Reply-To: References: <20070511131541.992688403@chello.nl> <20070511132321.895740140@chello.nl> <20070511093108.495feb70.akpm@linux-foundation.org> <20070511110522.ed459635.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Andi Kleen Cc: Christoph Lameter , Peter Zijlstra , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Oleg Nesterov , Ingo Molnar , Thomas Gleixner , Nick Piggin List-ID: On 12 May 2007 20:55:28 +0200 Andi Kleen wrote: > Andrew Morton writes: > > > On Fri, 11 May 2007 10:07:17 -0700 (PDT) > > Christoph Lameter wrote: > > > > > On Fri, 11 May 2007, Andrew Morton wrote: > > > > > > > yipes. percpu_counter_sum() is expensive. > > > > > > Capable of triggering NMI watchdog on 4096+ processors? > > > > Well. That would be a millisecond per cpu which sounds improbable. And > > we'd need to be calling it under local_irq_save() which we presently don't. > > And nobody has reported any problems against the existing callsites. > > > > But it's no speed demon, that's for sure. > > There is one possible optimization for this I did some time ago. You don't really > need to sum all over the possible map, but only all CPUs that were ever > online. But this only helps on systems where the possible map is bigger > than online map in the common case. But that shouldn't be the case anymore on x86 > -- it just used to be. If it's true on some other architectures it might > be still worth it. > hm, yeah. We could put a cpumask in percpu_counter, initialise it to cpu_possible_map. Then, those callsites which have hotplug notifiers can call into new percpu_counter functions which clear and set bits in that cpumask and which drain percpu_counter.counts[cpu] into percpu_counter.count. And percpu_counter_sum() gets taught to do for_each_cpu_mask(fbc->cpumask). -- 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