Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754557Ab3IXQvo (ORCPT ); Tue, 24 Sep 2013 12:51:44 -0400 Received: from merlin.infradead.org ([205.233.59.134]:53050 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754537Ab3IXQvm (ORCPT ); Tue, 24 Sep 2013 12:51:42 -0400 Date: Tue, 24 Sep 2013 18:51:21 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Paul McKenney , Thomas Gleixner , Steven Rostedt Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() Message-ID: <20130924165121.GQ9326@twins.programming.kicks-ass.net> References: <1378805550-29949-1-git-send-email-mgorman@suse.de> <1378805550-29949-38-git-send-email-mgorman@suse.de> <20130917143003.GA29354@twins.programming.kicks-ass.net> <20130917162050.GK22421@suse.de> <20130917164505.GG12926@twins.programming.kicks-ass.net> <20130918154939.GZ26785@twins.programming.kicks-ass.net> <20130919143241.GB26785@twins.programming.kicks-ass.net> <20130923175052.GA20991@redhat.com> <20130924123821.GT12926@twins.programming.kicks-ass.net> <20130924160359.GA2739@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130924160359.GA2739@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2081 Lines: 69 On Tue, Sep 24, 2013 at 06:03:59PM +0200, Oleg Nesterov wrote: > On 09/24, Peter Zijlstra wrote: > > > > +static inline void get_online_cpus(void) > > +{ > > + might_sleep(); > > + > > + if (current->cpuhp_ref++) { > > + barrier(); > > + return; > > I don't undestand this barrier()... we are going to return if we already > hold the lock, do we really need it? > > The same for put_online_cpus(). to make {get,put}_online_cpus() always behave like per-cpu lock sections. I don't think its ever 'correct' for loads/stores to escape the section, even if not strictly harmful. > > +void __get_online_cpus(void) > > { > > - if (cpu_hotplug.active_writer == current) > > + if (cpuhp_writer_task == current) > > return; > > Probably it would be better to simply inc/dec ->cpuhp_ref in > cpu_hotplug_begin/end and remove this check here and in > __put_online_cpus(). Oh indeed! > > + if (atomic_dec_and_test(&cpuhp_waitcount) && cpuhp_writer_task) > > + cpuhp_writer_wake(); > > cpuhp_writer_wake() here and in __put_online_cpus() looks racy... Yeah it is. Paul already said. > But, Peter, the main question is, why this is better than > percpu_rw_semaphore performance-wise? (Assuming we add > task_struct->cpuhp_ref). > > If the writer is pending, percpu_down_read() does > > down_read(&brw->rw_sem); > atomic_inc(&brw->slow_read_ctr); > __up_read(&brw->rw_sem); > > is it really much worse than wait_event + atomic_dec_and_test? > > And! please note that with your implementation the new readers will > be likely blocked while the writer sleeps in synchronize_sched(). > This doesn't happen with percpu_rw_semaphore. Good points both, no I don't think there's a significant performance gap there. I'm still hoping we can come up with something better though :/ I don't particularly like either. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/