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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 00EF7C352A3 for ; Tue, 11 Feb 2020 15:06:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C8290206D7 for ; Tue, 11 Feb 2020 15:06:14 +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="aQeCQQp0" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729157AbgBKPGO (ORCPT ); Tue, 11 Feb 2020 10:06:14 -0500 Received: from merlin.infradead.org ([205.233.59.134]:43572 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727111AbgBKPGN (ORCPT ); Tue, 11 Feb 2020 10:06:13 -0500 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; bh=kOVM19xgvEpzNbdc34dGz9f33pbISrxSA6jBH/Bi6So=; b=aQeCQQp0OxFOcoJ8vnnGz7DEKt oR7osXGRsXoca1k5+TmdiEJvRabVb6ElkP0ZmZrNG2BXx12YaCVHSVPuSPM59TaNmahz3nYnk+G3h bkrs1CrjM4b9Xrp8zOjk9PJZDQv4SGnm+HdsqQRNXl1aiAyCaHiX5GxkYjDN7n5VGAtwR3ZCy3vYN VUnK89EVOl+KiBNcmNZaDt9P+UMam1Tf0sWubBWzmveYa5B1PsT074ln+8T62ZpIMe+TjMR64cDte wMpCjFR1KytcQ3gi3AepbDSQrKrH0NntQdIJli72rwrftYJvCrL7O1ezIwbwZ4gNrIkDTC11uAkGE JrKESysQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1j1X6V-0006JB-RU; Tue, 11 Feb 2020 15:05:36 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 5450430066E; Tue, 11 Feb 2020 16:03:44 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id F2D092B920EF1; Tue, 11 Feb 2020 16:05:32 +0100 (CET) Date: Tue, 11 Feb 2020 16:05:32 +0100 From: Peter Zijlstra To: Steven Rostedt Cc: LKML , Ingo Molnar , "Joel Fernandes (Google)" , Greg Kroah-Hartman , "Gustavo A. R. Silva" , Thomas Gleixner , "Paul E. McKenney" , Josh Triplett , Mathieu Desnoyers , Lai Jiangshan Subject: Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook Message-ID: <20200211150532.GU14914@hirez.programming.kicks-ass.net> References: <20200210170643.3544795d@gandalf.local.home> <20200211114954.GK14914@hirez.programming.kicks-ass.net> <20200211090503.68c0d70f@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200211090503.68c0d70f@gandalf.local.home> 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 Tue, Feb 11, 2020 at 09:05:03AM -0500, Steven Rostedt wrote: > On Tue, 11 Feb 2020 12:49:54 +0100 > Peter Zijlstra wrote: > > > On Mon, Feb 10, 2020 at 05:06:43PM -0500, Steven Rostedt wrote: > > > + if (!rcu_watching) { \ > > > + /* Can not use RCU if rcu is not watching and in NMI */ \ > > > + if (in_nmi()) \ > > > + return; \ > > > + rcu_irq_enter_irqson(); \ > > > + } \ > > > > I saw the same weirdness in __trace_stack(), and I'm confused by it. > > > > How can we ever get to: in_nmi() && !rcu_watching() ? That should be a > > BUG. In particular, nmi_enter() has rcu_nmi_enter(). > > > > Paul, can that really happen? > > The stack tracer connects to the function tracer and is called at all > the places that function tracing can be called from. As I like being > able to trace RCU internal functions (especially as they are complex), > I don't want to set them all to notrace. But, for callbacks that > require RCU to be watching, we need this check, because there's some > internal state that we can be in an NMI and RCU is not watching (as > there's some places in nmi_enter that can be traced!). > > And if we are tracing preempt_enable and preempt_disable (as Joel added > trace events there), it may be the case for trace events too. Bloody hell; what a trainwreck. Luckily there's comments around that explain this! So we haz: | #define nmi_enter() \ | do { \ | arch_nmi_enter(); \ arm64 only, lets ignore for now | printk_nmi_enter(); \ notrace | lockdep_off(); \ notrace | ftrace_nmi_enter(); \ !notrace !!! | BUG_ON(in_nmi()); \ | preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);\ lets make this __preempt_count_add() ASAP ! | rcu_nmi_enter(); \ are you _really_ sure you want to go trace that ?!? | trace_hardirq_enter(); \ | } while (0)