From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753992Ab2GWHul (ORCPT ); Mon, 23 Jul 2012 03:50:41 -0400 Received: from orion.tchmachines.com ([208.76.84.200]:52946 "EHLO orion.tchmachines.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753969Ab2GWHuk (ORCPT ); Mon, 23 Jul 2012 03:50:40 -0400 Message-ID: <1343029838.6692.26.camel@vlad> Subject: Re: [RFC] optimize the locking in the rebalance_domains() From: Vlad Zolotarov To: Namhyung Kim Cc: Ingo Molnar , linux-kernel@vger.kernel.org, "Shai Fultheim (Shai@ScaleMP.com)" Date: Mon, 23 Jul 2012 10:50:38 +0300 In-Reply-To: <87fw8j6wnw.fsf@sejong.aot.lge.com> References: <1342974235.6692.20.camel@vlad> <87fw8j6wnw.fsf@sejong.aot.lge.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - orion.tchmachines.com X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - scalemp.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2012-07-23 at 11:25 +0900, Namhyung Kim wrote: > Hi, Vlad > > On Sun, 22 Jul 2012 19:23:55 +0300, Vlad Zolotarov wrote: > > Ingo, we've noticed that rebalance_domains() will try to take a lock > > every time it's called (every jiffy) if SD_SERIALIZE is set (which is a > > default configuration). This is done regardless the fact that maybe > > there hasn't passed enough time since the last rebalancing in which case > > there is no need to take a lock the first place. > > > > The above creates a heavy false sharing problem on the "balancing" > > spin-lock on large SMP systems: try_lock() is implemented with an > > (atomic) xchng instruction which invalidates the cache line "balancing" > > belongs to and therefore creates an intensive cross-NUMA-nodes traffic. > > > > The below patch will minimize the above phenomena to the time slots it's > > really needed, namely when the "interval" has really passed. > > > > Pls., comment. > > > > thanks, > > vlad > > > > --- > > kernel/sched/fair.c | 20 +++++++++++--------- > > 1 file changed, 11 insertions(+), 9 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index c099cc6..6777d38 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -4689,6 +4689,9 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle) > > interval = msecs_to_jiffies(interval); > > interval = clamp(interval, 1UL, max_load_balance_interval); > > > > + if (!time_after_eq(jiffies, sd->last_balance + interval)) > > + goto out; > > + > > First line looks like white-space-damaged. Looks like it. ;) Thanks for catching. I'll surely fix it if we get to posting the patch for applying. > Anyway, wouldn't it be better using time_before() here? Sure. I'll fix it as well. However I'd like to hear what u and other people on the mailing list think about the idea in general. Thanks, vlad > > Thanks, > Namhyung > > > > need_serialize = sd->flags & SD_SERIALIZE; > > > > if (need_serialize) { > > @@ -4696,16 +4699,15 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle) > > goto out; > > } > > > > - if (time_after_eq(jiffies, sd->last_balance + interval)) { > > - if (load_balance(cpu, rq, sd, idle, &balance)) { > > - /* > > - * We've pulled tasks over so either we're no > > - * longer idle. > > - */ > > - idle = CPU_NOT_IDLE; > > - } > > - sd->last_balance = jiffies; > > + if (load_balance(cpu, rq, sd, idle, &balance)) { > > + /* > > + * We've pulled tasks over so either we're no > > + * longer idle. > > + */ > > + idle = CPU_NOT_IDLE; > > } > > + sd->last_balance = jiffies; > > + > > if (need_serialize) > > spin_unlock(&balancing); > > out: