From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Date: Tue, 11 Dec 2012 06:07:06 -0800 Message-ID: <20121211140706.GB7084@htj.dyndns.org> References: <20121207173702.27305.1486.stgit@srivatsabhat.in.ibm.com> <20121207173759.27305.84316.stgit@srivatsabhat.in.ibm.com> <20121209191437.GA2816@redhat.com> <50C4EB79.5050203@linux.vnet.ibm.com> <20121209202234.GA5793@redhat.com> <50C564ED.9090803@linux.vnet.ibm.com> <20121210172410.GA28479@redhat.com> <50C73192.9010905@linux.vnet.ibm.com> <20121211134758.GA7084@htj.dyndns.org> <50C73CE5.1080201@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-pb0-f46.google.com ([209.85.160.46]:45715 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753771Ab2LKOHN (ORCPT ); Tue, 11 Dec 2012 09:07:13 -0500 Content-Disposition: inline In-Reply-To: <50C73CE5.1080201@linux.vnet.ibm.com> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: "Srivatsa S. Bhat" Cc: Oleg Nesterov , 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, 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 Hello, On Tue, Dec 11, 2012 at 07:32:13PM +0530, Srivatsa S. Bhat wrote: > On 12/11/2012 07:17 PM, Tejun Heo wrote: > > Hello, Srivatsa. > > > > On Tue, Dec 11, 2012 at 06:43:54PM +0530, Srivatsa S. Bhat wrote: > >> This approach (of using synchronize_sched()) also looks good. It is simple, > >> yet effective, but unfortunately inefficient at the writer side (because > >> he'll have to wait for a full synchronize_sched()). > > > > While synchornize_sched() is heavier on the writer side than the > > originally posted version, it doesn't stall the whole machine and > > wouldn't introduce latencies to others. Shouldn't that be enough? > > > > Short answer: Yes. But we can do better, with almost comparable code > complexity. So I'm tempted to try that out. > > Long answer: > Even in the synchronize_sched() approach, we still have to identify the > readers who need to be converted to use the new get/put_online_cpus_atomic() > APIs and convert them. Then, if we can come up with a scheme such that > the writer has to wait only for those readers to complete, then why not? > > If such a scheme ends up becoming too complicated, then I agree, we > can use synchronize_sched() itself. (That's what I meant by saying that > we'll use this as a fallback). > > But even in this scheme which uses synchronize_sched(), we are > already half-way through (we already use 2 types of sync schemes - > counters and rwlocks). Just a little more logic can get rid of the > unnecessary full-wait too.. So why not give it a shot? It's not really about the code complexity but making the reader side as light as possible. Please keep in mind that reader side is still *way* more hotter than the writer side. Before, the writer side was heavy to the extent which causes noticeable disruptions on the whole system and I think that's what we're trying to hunt down here. If we can shave of memory barriers from reader side by using synchornized_sched() on writer side, that is the *better* result, not worse. Thanks. -- tejun