From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 063F6C43461 for ; Tue, 15 Sep 2020 18:38:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BADEB20732 for ; Tue, 15 Sep 2020 18:38:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1600195088; bh=8Bkz/AaoLuYq/JpMcx55TXM/Y/n9rpfhHksMXOG/X/k=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=aWhSsubHrcG2blDJPrNIAqxkQbHMHXHXB614dkV36Z3OAOcvszTv0AXzHLM264coM nCY9n1YZdrrnOiA+Fo4r5O1HKms0ncnl0Axf8tnjmUZs1Wl6buN3Qx+MAAquBUmI9w 8nPS6UhQVu7gQjewtNTBgOEi5I/uoNE2Pdkym8u8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727925AbgIOSiG (ORCPT ); Tue, 15 Sep 2020 14:38:06 -0400 Received: from mail.kernel.org ([198.145.29.99]:37282 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727411AbgIOSHC (ORCPT ); Tue, 15 Sep 2020 14:07:02 -0400 Received: from paulmck-ThinkPad-P72.home (unknown [50.45.173.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 27DEA20738; Tue, 15 Sep 2020 18:06:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1600193174; bh=8Bkz/AaoLuYq/JpMcx55TXM/Y/n9rpfhHksMXOG/X/k=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=VzH21k8tWsnMRpcWFzTngBmcAX87k4b5rRKcDvUNx9Csbri5WH2TKN3PhpZ7VLLEu zLzEmxTr6CPzedJhRuwC2/eimadj9ycQi9x0ONmLOu6TazjI+RDOh2w4ggIyLxx3k/ 6MApWg/vAPsmLwxSf8HnJgdDkd8zNU0wX0ISXWVk= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id D2C133522718; Tue, 15 Sep 2020 11:06:13 -0700 (PDT) Date: Tue, 15 Sep 2020 11:06:13 -0700 From: "Paul E. McKenney" To: "Zhang,Qiang" Cc: Joel Fernandes , Uladzislau Rezki , "josh@joshtriplett.org" , "rostedt@goodmis.org" , "mathieu.desnoyers@efficios.com" , Lai Jiangshan , "rcu@vger.kernel.org" , LKML Subject: Re: =?utf-8?B?5Zue5aSN?= =?utf-8?Q?=3A?= RCU: Question on force_qs_rnp Message-ID: <20200915180613.GQ29330@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200914194208.GA2579423@google.com> <20200914205642.GE29330@paulmck-ThinkPad-P72> <20200915034139.GK29330@paulmck-ThinkPad-P72> <1079509d-c474-42bd-44e9-18cfa948fbae@windriver.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1079509d-c474-42bd-44e9-18cfa948fbae@windriver.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 15, 2020 at 01:16:39PM +0800, Zhang,Qiang wrote: > > > On 9/15/20 11:41 AM, Paul E. McKenney wrote: > > On Tue, Sep 15, 2020 at 03:18:23AM +0000, Zhang, Qiang wrote: > > > > > > > > > ________________________________________ > > > 发件人: Paul E. McKenney > > > 发送时间: 2020年9月15日 4:56 > > > 收件人: Joel Fernandes > > > 抄送: Zhang, Qiang; Uladzislau Rezki; josh@joshtriplett.org; rostedt@goodmis.org; mathieu.desnoyers@efficios.com; Lai Jiangshan; rcu@vger.kernel.org; LKML > > > 主题: Re: RCU: Question on force_qs_rnp > > > > > > On Mon, Sep 14, 2020 at 03:42:08PM -0400, Joel Fernandes wrote: > > > > On Mon, Sep 14, 2020 at 07:55:18AM +0000, Zhang, Qiang wrote: > > > > > Hello Paul > > > > > > > > > > I have some questions for you . > > > > > in force_qs_rnp func , if "f(rdp)" func return true we will call rcu_report_qs_rnp func > > > > > report a quiescent state for this rnp node, and clear grpmask form rnp->qsmask. > > > > > after that , can we make a check for this rnp->qsmask, if rnp->qsmask == 0, > > > > > we will check blocked readers in this rnp node, instead of jumping directly to the next node . > > > > > > > > Could you clarify what good is this going to do? What problem are you trying to > > > > address? > > > > > > > > You could have a task that is blocked in an RCU leaf node, but the > > > > force_qs_rnp() decided to call rcu_report_qs_rnp(). This is perfectly Ok. The > > > > CPU could be dyntick-idle and a quiescent state is reported. However, the GP > > > > must not end and the rcu leaf node should still be present in its parent > > > > intermediate nodes ->qsmask. In this case, the ->qsmask == 0 does not have > > > > any relevance. > > > > > > > > Or am I missing the point of the question? > > > > > > > Hello, Qiang, > > > > > > > Another way of making Joel's point is to say that the additional check > > > > you are asking for is already being done, but by rcu_report_qs_rnp(). > > > > > > > Thanx, Paul > > > > > > Hello Pual, Joel > > > > > > What I want to express is as follows : > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index 7623128d0020..beb554539f01 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -2622,6 +2622,11 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp)) > > > if (mask != 0) { > > > /* Idle/offline CPUs, report (releases rnp->lock). */ > > > rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags); > > > + raw_spin_lock_irqsave_rcu_node(rnp, flags); > > > + if (rnp->qsmask == 0 && rcu_preempt_blocked_readers_cgp(rnp)) > > > + rcu_initiate_boost(rnp, flags); > > > + else > > > + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > } else { > > > /* Nothing to do here, so just drop the lock. */ > > > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > > But in that case, why duplicate the code from rcu_initiate_boost()? > > > > Thanx, Paul > > > > Hello Paul > > When we force a qs for rnp, we first check the leaf node "rnp->qsmask" if it > is reached zero, will check if there are some blocked readers in this leaf > rnp node, if so we need to priority-boost blocked readers. > if not we will check cpu dyntick-idle and report leaf node qs, after this > leaf rnp node report qs, there is may be some blocked readers in this node, > should we also need to priority-boost blocked readers? Yes, but we will do that on the next time around, a few milliseconds later. And by that time, it is quite possible that the reader will have completed, which will save us from having to priority-boost it. Thanx, Paul