From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751573AbcGNS4M (ORCPT ); Thu, 14 Jul 2016 14:56:12 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:53099 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751262AbcGNS4J (ORCPT ); Thu, 14 Jul 2016 14:56:09 -0400 Date: Thu, 14 Jul 2016 20:56:05 +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 2/2] locking/percpu-rwsem: Introduce bias knob Message-ID: <20160714185605.GL30154@twins.programming.kicks-ass.net> References: <20160714182545.786693675@infradead.org> <20160714183022.336211504@infradead.org> <20160714184351.GA18388@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160714184351.GA18388@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 Thu, Jul 14, 2016 at 08:43:51PM +0200, Oleg Nesterov wrote: > On 07/14, Peter Zijlstra wrote: > > > > The current percpu-rwsem read side is entirely free of serializing > > instructions at the cost of having a synchronize_sched() in the write > > path. > > > > The latency of the synchronize_sched() is too high for some users > > (cgroups), so provide a __percpu_init_rwsem(.bias) argument to forgot > > this synchronize_sched() at the cost of forcing all readers into the > > slow path, which has serializing instructions. > > Oh well... I personally do not think this is what we want... Can't > we just add the stupid rcu_sync_enter() into cgroup_init() at least > for now? > Yes, this means the unnecessary .sync() at boot time, but > it will go away after cleanups I am going to send. Those would have to hit the same merge window though; some people (like Arjan) really care about boot times and hunt and kill people adding pointless delays.. > Because, again, we will probably want to change this bias dynamically. Hmm, how so?