From mboxrd@z Thu Jan 1 00:00:00 1970 From: "John Kacur" Subject: Re: [PATCH] Fix Bug messages Date: Thu, 31 Jul 2008 16:10:21 +0200 Message-ID: <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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "Sebastien Dugue" , "Chirag Jog" , "J?rgen Mell" , "Thomas Gleixner" , LKML , rt-users , "Steven Rostedt" , "Clark Williams" , "Josh Triplett" , "Timothy R. Chavez" To: "Peter Zijlstra" Return-path: Received: from nf-out-0910.google.com ([64.233.182.187]:20818 "EHLO nf-out-0910.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752808AbYGaOKW (ORCPT ); Thu, 31 Jul 2008 10:10:22 -0400 Received: by nf-out-0910.google.com with SMTP id d3so235412nfc.21 for ; Thu, 31 Jul 2008 07:10:21 -0700 (PDT) In-Reply-To: <1217512907.8157.91.camel@twins> Content-Disposition: inline Sender: linux-rt-users-owner@vger.kernel.org List-ID: 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?