From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Date: Thu, 13 Dec 2012 01:13:55 +0530 Message-ID: <50C8DE7B.8080708@linux.vnet.ibm.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> <20121212193646.GA29395@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e28smtp08.in.ibm.com ([122.248.162.8]:59179 "EHLO e28smtp08.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754076Ab2LLTp3 (ORCPT ); Wed, 12 Dec 2012 14:45:29 -0500 Received: from /spool/local by e28smtp08.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 13 Dec 2012 01:14:56 +0530 In-Reply-To: <20121212193646.GA29395@redhat.com> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Oleg Nesterov 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 On 12/13/2012 01:06 AM, Oleg Nesterov wrote: > 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. > Hmm.. And then the get/put_ on that CPU will increment/decrement the per-cpu refcount, but we don't care.. because we only need to ensure that they don't deadlock by taking the rwlock for read. This sounds great! Regards, Srivatsa S. Bhat