From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754650AbXIUACM (ORCPT ); Thu, 20 Sep 2007 20:02:12 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751975AbXIUAB6 (ORCPT ); Thu, 20 Sep 2007 20:01:58 -0400 Received: from e3.ny.us.ibm.com ([32.97.182.143]:43350 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751296AbXIUAB5 (ORCPT ); Thu, 20 Sep 2007 20:01:57 -0400 Date: Thu, 20 Sep 2007 17:01:55 -0700 From: "Paul E. McKenney" To: Dmitry Torokhov Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton , Nick Piggin Subject: Re: [RFC][PATCH 2/6] lockdep: validate rcu_dereference() vs rcu_read_lock() Message-ID: <20070921000155.GI9825@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20070919174857.GA11922@linux.vnet.ibm.com> <20070919214110.5d832f28@lappy> <20070919221349.2935f69d@lappy> <20070919231943.4b121361@lappy> <20070919234700.3b532dcd@lappy> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 20, 2007 at 01:31:35PM -0400, Dmitry Torokhov wrote: > On 9/19/07, Peter Zijlstra wrote: > > On Wed, 19 Sep 2007 17:29:09 -0400 "Dmitry Torokhov" > > wrote: > > > > > On 9/19/07, Peter Zijlstra wrote: > > > > On Wed, 19 Sep 2007 16:41:04 -0400 "Dmitry Torokhov" > > > > wrote: > > > > > > > > > > If the IRQ handler does rcu_read_lock(),unlock() and the i8042_stop() > > > > > > function does sync_rcu() instead of _sched(), it should be good again. > > > > > > It will not affect anything else than the task that calls _stop(). And > > > > > > even there the only change is that the sleep might be a tad longer. > > > > > > > > > > And the IRQ handler needs to do some extra job... Anyway, it looks -rt > > > > > breaks synchronize_sched() and needs to have it fixed: > > > > > > > > > > "/** > > > > > * synchronize_sched - block until all CPUs have exited any non-preemptive > > > > > * kernel code sequences. > > > > > * > > > > > * This means that all preempt_disable code sequences, including NMI and > > > > > * hardware-interrupt handlers, in progress on entry will have completed > > > > > * before this primitive returns." > > > > > > > > That still does as it says in -rt. Its just that the interrupt handler > > > > will be preemptible so the guarantees it gives are useless. > > > > > > Please note "... including NMI and hardware-interrupt handlers ..." > > > > -rt doesn't run interrupt handlers in hardware irq context anymore. > > OK, then what is the purpose of synchronize_sched() in -rt? To wait for all preempt-disable, irq-disable, hard-irq, and SMI/NMI code sequences to complete. > You really need to provide users with a replacement. There are several > drivers that use it and for example r8169 is not what you'd call a > 'low performer'. I did look at making a synchronize_all_irq() some time back, and all the approaches I came up with at the time were busted. But I just took another look, and I think I see a way to handle it. Either that, or I simply forgot the way in which this approach is broken... I will stare at is some more. > I guess I can switch i8042 to use synchronize_irq(). That still works > in -rt, doesn't it? That still leaves atkbd... Yep, looks that way to me. The only difference that I can see is that in -rt, concurrent synchronize_irq() calls on the same descriptor mean that the guy that gets there second has to wait for the next interrupt to happen. > > > > > > I find it curious that a driver that is 'low performant' and does not > > > > > > suffer lock contention pioneers locking schemes. I agree with > > > > > > optimizing, but this is not the place to push the envelope. > > > > > > > > > > Please realize that evey microsecond wasted on a 'low performant' > > > > > driver is taken from high performers and if we can help it why > > > > > shouldn't we? > > > > > > > > sure, but the cache eviction caused by running the driver will have > > > > more impact than the added rcu_read_{,un}lock() calls. > > > > > > Are you saying that adding rcu_read_{,un}lock() will help with cache > > > eviction? How? > > > > No, I'm saying that its noise compared to the cache eviction overhead > > it causes for others. > > What about udelay(10)? It is probably also a noise but we shoudl not > go and sprinkle it through drivers, should we? ;) Agreed! On the other hand, udelay(10) is more than two orders of magnitude slower than an rcu_read_lock() / rcu_read_unlock() round trip in -rt, and a full three orders of magnitude slower in CONFIG_PREEMPT. As for non-CONFIG_PREEMPT, well, "free is a very good price". ;-) Thanx, Paul