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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 AF278C433FF for ; Sun, 4 Aug 2019 14:49:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7703020844 for ; Sun, 4 Aug 2019 14:49:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="Wz3Uq0R7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726190AbfHDOtK (ORCPT ); Sun, 4 Aug 2019 10:49:10 -0400 Received: from merlin.infradead.org ([205.233.59.134]:43184 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726181AbfHDOtK (ORCPT ); Sun, 4 Aug 2019 10:49:10 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=Mtp+g7WpBJD26BgXFj0gCm0hISNxvHloZHwb4noPA5U=; b=Wz3Uq0R74klNGaZu8UGNMJB7/ 5QuAOTFIN3hvqlrVTOCAov5Cxt6LAww48N/CD5nk48MEnMN8hDGjlsbl9X02gy945lYsWp4WnCdac 1+kZg1V9wBj0Zv2rTK4Sck4yazQ1yiheMTR7V+VJ3OHFVm9571NMq/FSqBjkZT+ZRd+4E+GWGoWq6 pyugE2JiEBRZJaT73ax+ciahKlgbWY3syqdZliCP3WW9T8dRQl/4XVlMzqMImf7NzzlrnvF0kFXgI /DWMvKoI20lBAHgHayh1izWP/SYiwC+H12s43xwNA1m/jaYZ4klJlrnAaZIJKv4kJutAamtmERqWK 3oGoFQYnA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1huHoM-0007Oo-Hk; Sun, 04 Aug 2019 14:48:38 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 9C63B20274D75; Sun, 4 Aug 2019 16:48:35 +0200 (CEST) Date: Sun, 4 Aug 2019 16:48:35 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs Message-ID: <20190804144835.GB2386@hirez.programming.kicks-ass.net> References: <20190802151435.GA1081@linux.ibm.com> <20190802151501.13069-14-paulmck@linux.ibm.com> <20190804144317.GF2349@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190804144317.GF2349@hirez.programming.kicks-ass.net> 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 04, 2019 at 04:43:17PM +0200, Peter Zijlstra wrote: > On Fri, Aug 02, 2019 at 08:15:01AM -0700, Paul E. McKenney wrote: > > The multi_cpu_stop() function relies on the scheduler to gain control from > > whatever is running on the various online CPUs, including any nohz_full > > CPUs running long loops in kernel-mode code. Lack of the scheduler-clock > > interrupt on such CPUs can delay multi_cpu_stop() for several minutes > > and can also result in RCU CPU stall warnings. This commit therefore > > causes multi_cpu_stop() to enable the scheduler-clock interrupt on all > > online CPUs. > > This sounds wrong; should we be fixing sched_can_stop_tick() instead to > return false when the stop task is runnable? And even without that; I don't understand how we're not instantly preempted the moment we enqueue the stop task. Any enqueue, should go through check_preempt_curr() which will be an instant resched_curr() when we just woke the stop class.