From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751848AbcGOTrN (ORCPT ); Fri, 15 Jul 2016 15:47:13 -0400 Received: from merlin.infradead.org ([205.233.59.134]:48246 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751561AbcGOTrM (ORCPT ); Fri, 15 Jul 2016 15:47:12 -0400 Date: Fri, 15 Jul 2016 21:47:03 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, tj@kernel.org, paulmck@linux.vnet.ibm.com, john.stultz@linaro.org, dimitrysh@google.com, romlem@google.com, ccross@google.com, tkjos@google.com Subject: Re: [PATCH 1/2] locking/percpu-rwsem: Optimize readers and reduce global impact Message-ID: <20160715194703.GD3115@twins.programming.kicks-ass.net> References: <20160714182545.786693675@infradead.org> <20160714183022.272275102@infradead.org> <20160715163054.GA2995@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160715163054.GA2995@redhat.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 15, 2016 at 06:30:54PM +0200, Oleg Nesterov wrote: > On 07/14, Peter Zijlstra wrote: > > > > Currently the percpu-rwsem switches to (global) atomic ops while a > > writer is waiting; which could be quite a while and slows down > > releasing the readers. > > > > This patch cures this problem by ordering the reader-state vs > > reader-count (see the comments in __percpu_down_read() and > > percpu_down_write()). This changes a global atomic op into a full > > memory barrier, which doesn't have the global cacheline contention. > > I've applied this patch + another change you sent on top of it. > > Everything looks good to me except the __this_cpu_inc() in > __percpu_down_read(), > > > + __down_read(&sem->rw_sem); > > + __this_cpu_inc(*sem->read_count); > > + __up_read(&sem->rw_sem); > > Preemption is already enabled, don't we need this_cpu_inc() ? Ah indeed. This mistake is quite old it seems, good catch. > > -EXPORT_SYMBOL_GPL(percpu_up_write); > > +EXPORT_SYMBOL(percpu_up_write); > > and this one ;) I do not really care, but it seems you did this change > by accident. Yep, oops ;-) > Actually, I _think_ we can do some cleanups/improvements on top of this > change, but we can do this later. In particular, _perhaps_ we can avoid > the unconditional wakeup in __percpu_up_read(), but I am not sure and in > any case this needs another change. > > Reviewed-by: Oleg Nesterov Thanks!