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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 3D00BC43140 for ; Fri, 6 Sep 2019 17:26:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 152CD208C3 for ; Fri, 6 Sep 2019 17:26:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="g+kiLp3a" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2405963AbfIFR0q (ORCPT ); Fri, 6 Sep 2019 13:26:46 -0400 Received: from mail-pf1-f194.google.com ([209.85.210.194]:40924 "EHLO mail-pf1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731974AbfIFR0p (ORCPT ); Fri, 6 Sep 2019 13:26:45 -0400 Received: by mail-pf1-f194.google.com with SMTP id x127so4938030pfb.7 for ; Fri, 06 Sep 2019 10:26:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=tjTFPsf2DNxsjE2a8i9G461OudiUTTXZgFUmeTRYsOE=; b=g+kiLp3az9CyjTc6pxcyGvJcUmtfHu6/Pb78/cJdv6XEo9ckaguTCF/9/EagFdvnCo jPUDiZuxJze1xG/wj0Xrk0SxtxbYayWzf2xgBvWqusIPD8xk2NMphiwZmc1tkOkghM4r JgbY4nksDcpfoiJWmk9B7Sx0dn15waEhL5iHg= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=tjTFPsf2DNxsjE2a8i9G461OudiUTTXZgFUmeTRYsOE=; b=tmDaERgN67W+5IS300rc1uWJR7KrlIkEjIESqZxxpRv8Fnl63dxEDNWNx2Et0S3yyO uV6ToNZe7/cEqls/fZLXnUVc2DxfZAdxmh/eJR0LhUI0pLXIax7eNeT2uq3poVKQ+LxU gy+gqZFlOghA0lk9M2aPFVy8PqNbOeGObTlS3GQre+cijH+jdgoffWOCwbdto+Cxv/zx 3MKRl7IwejfmdoXkiqL6HAf3fXijpqYwRMdntFhFzgKu4pkpPACl6ExXqf0njd7ZYAFE a0I8OwBsPb7hKDhteLimJL8JQN3Pt1o+5+RODz1qbnJUnvw07+R8H4VnI/B7YQ5UkM+b uGIQ== X-Gm-Message-State: APjAAAU3UjO9bZg7xnDJVblqttpLB3kkJ8qVoG/B4Ui6cg4+Z96cFS5p Z4xi1q7LbGChxckmOp9Aqxt/Ag== X-Google-Smtp-Source: APXvYqxUjh8vYhfUkzzHghKc6iGyrAQB8idr1czO4uE7him1Zaxi0rWcC03ts8w3NLYgI4Sk7bXGFQ== X-Received: by 2002:a62:52d0:: with SMTP id g199mr12269924pfb.120.1567790804635; Fri, 06 Sep 2019 10:26:44 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id a12sm4974891pgv.48.2019.09.06.10.26.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Sep 2019 10:26:43 -0700 (PDT) Date: Fri, 6 Sep 2019 13:26:42 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Andy Lutomirski , Bjorn Helgaas , Ingo Molnar , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , Petr Mladek , "Rafael J. Wysocki" , rcu@vger.kernel.org, Steven Rostedt , Yafang Shao Subject: Re: [PATCH -rcu dev 1/2] Revert b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks counter") Message-ID: <20190906172642.GB40876@google.com> References: <20190904135420.GB240514@google.com> <20190904231308.GB4125@linux.ibm.com> <20190905153620.GG26466@google.com> <20190905164329.GT4125@linux.ibm.com> <20190906000137.GA224720@google.com> <20190906150806.GA11355@google.com> <20190906152144.GF4051@linux.ibm.com> <20190906152753.GA18734@linux.ibm.com> <20190906165751.GA40876@google.com> <20190906171646.GI4051@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190906171646.GI4051@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 06, 2019 at 10:16:47AM -0700, Paul E. McKenney wrote: > On Fri, Sep 06, 2019 at 12:57:51PM -0400, Joel Fernandes wrote: > > On Fri, Sep 06, 2019 at 08:27:53AM -0700, Paul E. McKenney wrote: > > > On Fri, Sep 06, 2019 at 08:21:44AM -0700, Paul E. McKenney wrote: > > > > On Fri, Sep 06, 2019 at 11:08:06AM -0400, Joel Fernandes wrote: > > > > > On Thu, Sep 05, 2019 at 08:01:37PM -0400, Joel Fernandes wrote: > > > > > [snip] > > > > > > > > > @@ -3004,7 +3007,7 @@ static int rcu_pending(void) > > > > > > > > > return 0; > > > > > > > > > > > > > > > > > > /* Is the RCU core waiting for a quiescent state from this CPU? */ > > > > > > > > > - if (rdp->core_needs_qs && !rdp->cpu_no_qs.b.norm) > > > > > > > > > + if (READ_ONCE(rdp->core_needs_qs) && !rdp->cpu_no_qs.b.norm) > > > > > > > > > return 1; > > > > > > > > > > > > > > > > > > /* Does this CPU have callbacks ready to invoke? */ > > > > > > > > > @@ -3244,7 +3247,6 @@ int rcutree_prepare_cpu(unsigned int cpu) > > > > > > > > > rdp->gp_seq = rnp->gp_seq; > > > > > > > > > rdp->gp_seq_needed = rnp->gp_seq; > > > > > > > > > rdp->cpu_no_qs.b.norm = true; > > > > > > > > > - rdp->core_needs_qs = false; > > > > > > > > > > > > > > > > How about calling the new hint-clearing function here as well? Just for > > > > > > > > robustness and consistency purposes? > > > > > > > > > > > > > > This and the next function are both called during a CPU-hotplug online > > > > > > > operation, so there is little robustness or consistency to be had by > > > > > > > doing it twice. > > > > > > > > > > > > Ok, sorry I missed you are clearing it below in the next function. That's > > > > > > fine with me. > > > > > > > > > > > > This patch looks good to me and I am Ok with merging of these changes into > > > > > > the original patch with my authorship as you mentioned. Or if you wanted to > > > > > > be author, that's fine too :) > > > > > > > > > > Paul, does it make sense to clear these urgency hints in rcu_qs() as well? > > > > > After all, we are clearing atleast one urgency hint there: the > > > > > rcu_read_unlock_special::need_qs bit. > > > > Makes sense. > > > > > > We certainly don't want to turn off the scheduling-clock interrupt until > > > > after the quiescent state has been reported to the RCU core. And it might > > > > still be useful to have a heavy quiescent state because the grace-period > > > > kthread can detect that. Just in case the CPU that just called rcu_qs() > > > > is slow about actually reporting that quiescent state to the RCU core. > > > > > > Hmmm... Should ->need_qs ever be cleared from FQS to begin with? > > > > I did not see the FQS loop clearing ->need_qs in the rcu_read_unlock_special > > union after looking for a few minutes. Could you clarify which path this? > > > > Or do you mean ->core_needs_qs? If so, I feel the FQS loop should clear it as > > I think your patch does, since the FQS loop is essentially doing heavy-weight > > RCU core processing right? > > > > Also, where does FQS loop clear rdp.cpu_no_qs? Shouldn't it clear that as > > well for the dyntick-idle CPUs? > > Synchronization? Didn't follow. Probably I am asking silly questions. There's too many hints so it is confusing. I will trace this out later and study it more. Doing by best, ;-) thanks, - Joel