From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755957AbbAPB6Z (ORCPT ); Thu, 15 Jan 2015 20:58:25 -0500 Received: from cdptpa-outbound-snat.email.rr.com ([107.14.166.227]:49184 "EHLO cdptpa-oedge-vip.email.rr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752711AbbAPB6Y (ORCPT ); Thu, 15 Jan 2015 20:58:24 -0500 Date: Thu, 15 Jan 2015 20:58:37 -0500 From: Steven Rostedt To: Tim Chen Cc: Peter Zijlstra , Ingo Molnar , Andi Kleen , Shawn Bohrer , Suruchi Kadu , Doug Nelson , linux-kernel@vger.kernel.org Subject: Re: [PATCH] Repost sched-rt: Reduce rq lock contention by eliminating locking of non-feasible target Message-ID: <20150115205837.22075a58@grimm.local.home> In-Reply-To: <1420570911.2399.4.camel@schen9-desk2.jf.intel.com> References: <1420570911.2399.4.camel@schen9-desk2.jf.intel.com> X-Mailer: Claws Mail 3.11.1 (GTK+ 2.24.25; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-RR-Connecting-IP: 107.14.168.118:25 X-Cloudmark-Score: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 06 Jan 2015 11:01:51 -0800 Tim Chen wrote: > Didn't get any response for this patch probably due to the holidays. > Reposting it as we will like to get it merged to help our database > workload. > > This patch added checks that prevent futile attempts to move rt tasks > to cpu with active tasks of equal or higher priority. This reduces > run queue lock contention and improves the performance of a well > known OLTP benchmark by 0.7%. > > Signed-off-by: Tim Chen > --- > kernel/sched/rt.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index ee15f5a..0e4382e 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -1337,7 +1337,8 @@ select_task_rq_rt(struct task_struct *p, int > cpu, int sd_flag, int flags) curr->prio <= p->prio)) { > int target = find_lowest_rq(p); > Please add a comment here that says something like: /* * Don't bother moving it if the destination CPU is * not running a lower priority task. */ > - if (target != -1) > + if (target != -1 && > + p->prio < cpu_rq(target)->rt.highest_prio.curr) > cpu = target; > } > rcu_read_unlock(); > @@ -1613,6 +1614,12 @@ static struct rq *find_lock_lowest_rq(struct > task_struct *task, struct rq *rq) break; > > lowest_rq = cpu_rq(cpu); > + > + if (lowest_rq->rt.highest_prio.curr <= task->prio) { > + /* target rq has tasks of equal or higher priority, > try again */ > + lowest_rq = NULL; > + continue; This should just break out and not try again. The reason for the other try again is because of the double_lock which can release the locks which can cause a process waiting for the lock to sneak in and change the priorities. But this case, a try again is highly unlikely to do anything differently (no locks are released) and just waste cycles. -- Steve > + } > > /* if the prio of this runqueue changed, try again */ > if (double_lock_balance(rq, lowest_rq)) {