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=-8.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 EFDC1C3A589 for ; Sun, 18 Aug 2019 22:32:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A4BFB21852 for ; Sun, 18 Aug 2019 22:32:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="mobHKqeQ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726107AbfHRWct (ORCPT ); Sun, 18 Aug 2019 18:32:49 -0400 Received: from mail-pf1-f194.google.com ([209.85.210.194]:38831 "EHLO mail-pf1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726088AbfHRWcs (ORCPT ); Sun, 18 Aug 2019 18:32:48 -0400 Received: by mail-pf1-f194.google.com with SMTP id o70so5973075pfg.5 for ; Sun, 18 Aug 2019 15:32:48 -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=3+t62dzDoveuxlx7LxDUcktLsRGM2iYfF3y6zujhsKQ=; b=mobHKqeQZ2IuRcoEghaxJf1kWA69qE6pHAcUyv1u0a0q7Vp5hKy+vieAlcjFX3YBHt UIqketDAiZh/OCibAfbq4ddI8TEEuGpTAZ5qX0gLl17O5voHpT1Xcd4kmlMukAASf4E+ 0AX4elQRRUxqB79QgfYkE9rYYibJ8vqQzVHhw= 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=3+t62dzDoveuxlx7LxDUcktLsRGM2iYfF3y6zujhsKQ=; b=MrIJOj7hbgrTdkqypqM9nywVDlo+T9KgNGANtAD2ZjEO96B4kV9WWZrkcg3KVtzvH6 EFlip41/rRSlKfuvDxSQI0moSOGmcVj/ZgpwiVbOXWieHpgqHWJ+hLb9YMzGjdQm4aj+ QODhKomol1KgBfbC6iqKyS7RV5SMnkDFuuI384+AB6WJsbLDx45/3AWeJ+l7t52O3HQZ 3ZbxQyi0yuTdt+OdkOaEePEAGvsLlrLO0EINA22ceNMEiKWqeIb25vu85tonPzoG8aJi 0AyAypWFjjExrqWutw5A1Vq31+WzZdpp+eLJpbGpdExaMAyNW2sjW+kjWJAvaZ/NgtEW dAEg== X-Gm-Message-State: APjAAAVPEbHykaESUv6m1Nhb9opTvdzIhu1BEbuKPgpdOTDTjxOx0w9n 0A1b41LJ2nx4uHWRjIIrzZJA2Q== X-Google-Smtp-Source: APXvYqzGIszjAJUSsQYnHE2gryQTajErvu2ajQPUWFyo5lqT1YNOmiEN4sR43L/pe1Boqq298b+8oA== X-Received: by 2002:a17:90a:f:: with SMTP id 15mr17863798pja.93.1566167567869; Sun, 18 Aug 2019 15:32:47 -0700 (PDT) Received: from localhost ([172.19.216.18]) by smtp.gmail.com with ESMTPSA id a26sm13166894pff.174.2019.08.18.15.32.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 18 Aug 2019 15:32:47 -0700 (PDT) Date: Sun, 18 Aug 2019 18:32:30 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock Message-ID: <20190818223230.GA143857@google.com> References: <20190818214948.GA134430@google.com> <20190818221210.GP28441@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190818221210.GP28441@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sun, Aug 18, 2019 at 03:12:10PM -0700, Paul E. McKenney wrote: > On Sun, Aug 18, 2019 at 05:49:48PM -0400, Joel Fernandes (Google) wrote: > > When we're in hard interrupt context in rcu_read_unlock_special(), we > > can still benefit from invoke_rcu_core() doing wake ups of rcuc > > threads when the !use_softirq parameter is passed. This is safe > > to do so because: > > > > 1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit > > introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe > > conditions in rcu_read_unlock_special()") by checking for the same in > > this patch. > > > > 2. in_irq() implies in_interrupt() which implies raising softirq will > > not do any wake ups. > > > > The rcuc thread which is awakened will run when the interrupt returns. > > > > We also honor 25102de ("rcu: Only do rcu_read_unlock_special() wakeups > > if expedited") thus doing the rcuc awakening only when none of the > > following are true: > > 1. Critical section is blocking an expedited GP. > > 2. A nohz_full CPU. > > If neither of these cases are true (exp == false), then the "else" block > > will run to do the irq_work stuff. > > > > This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify > > rcu_read_unlock_special() deferred wakeups") with an additional in_irq() > > check added. > > > > Signed-off-by: Joel Fernandes (Google) > > OK, I will bite... If it is safe to wake up an rcuc kthread, why > is it not safe to do raise_softirq()? Because raise_softirq should not be done and/or doesn't do anything if use_softirq == false. In fact, RCU_SOFTIRQ doesn't even existing if use_softirq == false. The "else if" condition of this patch uses for use_softirq. Or, did I miss your point? > And from the nit department, looks like some whitespace damage on the > comments. I will fix all of these in the change log, it was just a quick RFC I sent with the idea, tagged as RFC and not yet for merging. I should also remove the comment about " in_irq() implies in_interrupt() which implies raising softirq" from the changelog since this patch is only concerned with the rcuc kthread. thanks! - Joel > Thanx, Paul > > > --- > > v1->v2: Some minor character encoding issues in changelog corrected. > > > > Note that I am still testing this patch, but I sent an early RFC for your > > feedback. Thanks! > > > > kernel/rcu/tree_plugin.h | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 2defc7fe74c3..f4b3055026dc 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -621,6 +621,11 @@ static void rcu_read_unlock_special(struct task_struct *t) > > // Using softirq, safe to awaken, and we get > > // no help from enabling irqs, unlike bh/preempt. > > raise_softirq_irqoff(RCU_SOFTIRQ); > > + } else if (exp && in_irq() && !use_softirq && > > + !t->rcu_read_unlock_special.b.deferred_qs) { > > + // Safe to awaken rcuc kthread which will be > > + // scheduled in from the interrupt return path. > > + invoke_rcu_core(); > > } else { > > // Enabling BH or preempt does reschedule, so... > > // Also if no expediting or NO_HZ_FULL, slow is OK. > > -- > > 2.23.0.rc1.153.gdeed80330f-goog > > >