From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751534Ab0KYDlb (ORCPT ); Wed, 24 Nov 2010 22:41:31 -0500 Received: from cn.fujitsu.com ([222.73.24.84]:50714 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751331Ab0KYDl3 (ORCPT ); Wed, 24 Nov 2010 22:41:29 -0500 Message-ID: <4CEDDB2A.2020807@cn.fujitsu.com> Date: Thu, 25 Nov 2010 11:42:34 +0800 From: Lai Jiangshan User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.9) Gecko/20100423 Thunderbird/3.0.4 MIME-Version: 1.0 To: Frederic Weisbecker CC: "Paul E. McKenney" , LKML , Thomas Gleixner , Peter Zijlstra , Steven Rostedt , Ingo Molnar Subject: Re: [PATCH 0/2] rcu: Fix series of spurious RCU softirqs References: <1290558673-23580-1-git-send-crap-fweisbec@gmail.com> In-Reply-To: <1290558673-23580-1-git-send-crap-fweisbec@gmail.com> X-MIMETrack: Itemize by SMTP Server on mailserver/fnst(Release 8.5.1FP4|July 25, 2010) at 2010-11-25 11:41:44, Serialize by Router on mailserver/fnst(Release 8.5.1FP4|July 25, 2010) at 2010-11-25 11:41:45, Serialize complete at 2010-11-25 11:41:45 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/24/2010 08:31 AM, Frederic Weisbecker wrote: > Hi, > > I've observed some not so unfrequent series of spurious rcu > softirqs, sometimes happening at each ticks for a random > while. > > These patches aims at fixing them. > > Thanks. > > Frederic Weisbecker (2): > rcu: Don't chase unnecessary quiescent states after extended grace periods > rcu: Stop checking quiescent states after grace period completion from remote > If we ensure rdp->gpnum >= rdp->completed is always true, the problems as you described will not be existed. Or maybe I misunderstand you. rdp->gpnum >= rdp->completed is a very important guarantee I think. (In my RCURING, it is guaranteed.) I'm afraid there are some other problems still hidden if it is not guaranteed. so I recommend: (code is better than words) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index d5bc439..af4e87a 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -648,6 +648,13 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat /* Remember that we saw this grace-period completion. */ rdp->completed = rnp->completed; + + /* Ensure ->gpnum >= ->completed after NO_HZ */ + if (unlikely(rnp->completed - rdp->gpnum > 0 + || rdp->gpnum - rnp->gpnum > 0)) { + rdp->gpnum = rnp->completed; + rdp->qs_pending = 0; + } } }