From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758654AbaGQXnp (ORCPT ); Thu, 17 Jul 2014 19:43:45 -0400 Received: from mail-yh0-f53.google.com ([209.85.213.53]:45242 "EHLO mail-yh0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754542AbaGQXnn (ORCPT ); Thu, 17 Jul 2014 19:43:43 -0400 Message-ID: <53C85FCA.6080107@gmail.com> Date: Thu, 17 Jul 2014 19:44:10 -0400 From: Pranith Kumar User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Christoph Lameter CC: rdunlap@infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] doc: Add remote CPU access details and others to this_cpu_ops.txt References: <1405552141-8506-1-git-send-email-bobby.prani@gmail.com> <53C7D93B.4090006@gmail.com> <53C7E234.3020503@gmail.com> <53C7E5C8.9070400@gmail.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/17/2014 11:26 AM, Christoph Lameter wrote: > On Thu, 17 Jul 2014, Pranith Kumar wrote: > >> I can mention that IPI is preferable. What is that you don't want mentioned? atomic_t? > > Definitely not as an example. atomic_t in per cpu areas is self > contradicting. The per cpu area is exclusively for that processor whereas > an atomic_t is supposed to be accessed from multiple processors. > >>> Remote percpu updates are extremely rare events. If the cpu is idle/asleep >>> then usually no updates are needed because no activity is occurring on >>> that cpu. >>> >> >> Yes, -usually- that is the case. But we are talking about the extreme rare event >> where we need to update some remote CPU`s per-cpu data without waking it up from >> sleep/idle. How do you suggest we handle this? I don't think suggesting not to >> use per-cpu areas because of this is a good idea, since we lose a lot of >> performance in the most common cases. > > If you modify a percpu area then that is usually done because that cpu > needs to take some action. An IPI is fine. > > Otherwise yes I would suggest not use a percpu area but a separate data > structure for synchronization. > Yes, I will add this information to the doc. Thanks! -- Pranith