From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754857Ab2LLTg7 (ORCPT ); Wed, 12 Dec 2012 14:36:59 -0500 Received: from mx1.redhat.com ([209.132.183.28]:18451 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754212Ab2LLTg6 (ORCPT ); Wed, 12 Dec 2012 14:36:58 -0500 Date: Wed, 12 Dec 2012 20:36:46 +0100 From: Oleg Nesterov To: "Srivatsa S. Bhat" Cc: tglx@linutronix.de, peterz@infradead.org, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org, vincent.guittot@linaro.org, tj@kernel.org, sbw@mit.edu, amit.kucheria@linaro.org, rostedt@goodmis.org, rjw@sisk.pl, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Message-ID: <20121212193646.GA29395@redhat.com> References: <20121211140314.23621.64088.stgit@srivatsabhat.in.ibm.com> <20121211140358.23621.97011.stgit@srivatsabhat.in.ibm.com> <20121212171720.GA22289@redhat.com> <50C8C4A5.4080104@linux.vnet.ibm.com> <20121212180248.GA24882@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121212180248.GA24882@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/12, Oleg Nesterov wrote: > > On 12/12, Srivatsa S. Bhat wrote: > > > > On 12/12/2012 10:47 PM, Oleg Nesterov wrote: > > > > > > Why it needs to be per-cpu? It can be global and __read_mostly to avoid > > > the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal > > > into a single cacheline... > > > > Even I realized this (that we could use a global) after posting out the > > series.. But do you think that it would be better to retain the per-cpu > > variant itself, due to the cache effects? > > I don't really know, up to you. This was the question ;) But perhaps there is another reason to make it per-cpu... It seems we can avoid cpu_hotplug.active_writer == current check in get/put. take_cpu_down() can clear this_cpu(writer_signal) right after it takes hotplug_rwlock for writing. It runs with irqs and preemption disabled, nobody else will ever look at writer_signal on its CPU. Oleg.