From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756870AbXISVss (ORCPT ); Wed, 19 Sep 2007 17:48:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751658AbXISVsj (ORCPT ); Wed, 19 Sep 2007 17:48:39 -0400 Received: from canuck.infradead.org ([209.217.80.40]:35577 "EHLO canuck.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751540AbXISVsj (ORCPT ); Wed, 19 Sep 2007 17:48:39 -0400 Date: Wed, 19 Sep 2007 23:47:00 +0200 From: Peter Zijlstra To: "Dmitry Torokhov" Cc: paulmck@linux.vnet.ibm.com, 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: <20070919234700.3b532dcd@lappy> In-Reply-To: References: <20070919104125.286538000@chello.nl> <20070919173249.GE8666@linux.vnet.ibm.com> <20070919174857.GA11922@linux.vnet.ibm.com> <20070919214110.5d832f28@lappy> <20070919221349.2935f69d@lappy> <20070919231943.4b121361@lappy> X-Mailer: Claws Mail 3.0.0 (GTK+ 2.11.6; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org 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. > > > > > > 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.