Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756578AbYGaOSw (ORCPT ); Thu, 31 Jul 2008 10:18:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754682AbYGaOSl (ORCPT ); Thu, 31 Jul 2008 10:18:41 -0400 Received: from casper.infradead.org ([85.118.1.10]:41021 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753002AbYGaOSk (ORCPT ); Thu, 31 Jul 2008 10:18:40 -0400 Subject: Re: [PATCH] Fix Bug messages From: Peter Zijlstra To: John Kacur Cc: Sebastien Dugue , Chirag Jog , J?rgen Mell , Thomas Gleixner , LKML , rt-users , Steven Rostedt , Clark Williams , Josh Triplett , "Timothy R. Chavez" In-Reply-To: <520f0cf10807310710i85e5a2u8e5f227bc2b82051@mail.gmail.com> References: <200807301101.32417.j.mell@t-online.de> <20080730171842.GB3420@linux.vnet.ibm.com> <20080731100023.0221ec2b@bull.net> <520f0cf10807310313q45599221q3db1b6fd7e7c722f@mail.gmail.com> <20080731132336.362bd487@bull.net> <520f0cf10807310649l1083e29bmf0f704e13dee96c@mail.gmail.com> <1217512907.8157.91.camel@twins> <520f0cf10807310710i85e5a2u8e5f227bc2b82051@mail.gmail.com> Content-Type: text/plain Date: Thu, 31 Jul 2008 16:18:00 +0200 Message-Id: <1217513880.8157.96.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3419 Lines: 88 On Thu, 2008-07-31 at 16:10 +0200, John Kacur wrote: > On Thu, Jul 31, 2008 at 4:01 PM, Peter Zijlstra wrote: > > On Thu, 2008-07-31 at 15:49 +0200, John Kacur wrote: > >> Signed-off-by: John Kacur > >> Index: linux-2.6.26-rt1/net/core/sock.c > >> =================================================================== > >> --- linux-2.6.26-rt1.orig/net/core/sock.c > >> +++ linux-2.6.26-rt1/net/core/sock.c > >> @@ -1986,11 +1986,12 @@ static __init int net_inuse_init(void) > >> > >> core_initcall(net_inuse_init); > >> #else > >> -static DEFINE_PER_CPU(struct prot_inuse, prot_inuse); > >> +static DEFINE_PER_CPU_LOCKED(struct prot_inuse, prot_inuse); > >> > >> void sock_prot_inuse_add(struct net *net, struct proto *prot, int val) > >> { > >> - __get_cpu_var(prot_inuse).val[prot->inuse_idx] += val; > >> + int cpu = 0; > >> + __get_cpu_var_locked(prot_inuse, cpu).val[prot->inuse_idx] += val; > >> } > >> EXPORT_SYMBOL_GPL(sock_prot_inuse_add); > >> > >> @@ -2000,7 +2001,7 @@ int sock_prot_inuse_get(struct net *net, > >> int res = 0; > >> > >> for_each_possible_cpu(cpu) > >> - res += per_cpu(prot_inuse, cpu).val[idx]; > >> + res += per_cpu_var_locked(prot_inuse, cpu).val[idx]; > >> > >> return res >= 0 ? res : 0; > >> } > > > > This doesn't look good. You declare it as a PER_CPU_LOCKED, but then > > never use the extra lock to synchronize data. > > > > Given that sock_proc_inuse_get() is a racy read anyway, the 'right' fix > > would be to do something like: > > > > diff --git a/net/core/sock.c b/net/core/sock.c > > index 91f8bbc..5a8ace4 100644 > > --- a/net/core/sock.c > > +++ b/net/core/sock.c > > @@ -1941,8 +1941,9 @@ static DECLARE_BITMAP(proto_inuse_idx, PROTO_INUSE_NR); > > #ifdef CONFIG_NET_NS > > void sock_prot_inuse_add(struct net *net, struct proto *prot, int val) > > { > > - int cpu = smp_processor_id(); > > + int cpu = get_cpu(); > > per_cpu_ptr(net->core.inuse, cpu)->val[prot->inuse_idx] += val; > > + put_cpu(); > > } > > EXPORT_SYMBOL_GPL(sock_prot_inuse_add); > > > > @@ -1988,7 +1989,9 @@ static DEFINE_PER_CPU(struct prot_inuse, prot_inuse); > > > > void sock_prot_inuse_add(struct net *net, struct proto *prot, int val) > > { > > - __get_cpu_var(prot_inuse).val[prot->inuse_idx] += val; > > + int cpu = get_cpu(); > > + per_cpu(prot_inuse, cpu).val[prot->inuse_idx] += val; > > + put_cpu(); > > } > > EXPORT_SYMBOL_GPL(sock_prot_inuse_add); > > > > This disables preemption, but only for a very short time - so it doesn't > > hurt the preempt-latency. > > > > The alternative is to take a lock, do the inc, and drop the lock again, > > which is much more expensive. > > > > > > Cool, thanks for the quick feedback. What kind of criteria are used to > decide between disabling preemption for a short time, or using the > more expensive lock? Basically total cost of the operation.. in this case the cost of taking the lock utterly dwarfs the cost of the operation. And since its Real-Time we're talking about, its the WCET of the operation that counts. -- 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/