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 03B00C3A5A7 for ; Wed, 4 Sep 2019 08:19:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CFD9622DBF for ; Wed, 4 Sep 2019 08:19:35 +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="JVZzJAjU" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729253AbfIDITf (ORCPT ); Wed, 4 Sep 2019 04:19:35 -0400 Received: from merlin.infradead.org ([205.233.59.134]:60244 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726486AbfIDITe (ORCPT ); Wed, 4 Sep 2019 04:19:34 -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=ZZNaQ4LXZ7GAaOEQI0TFwLUNueXowwuFXVaKHgUrpds=; b=JVZzJAjUvutqZpA1EnnBt9GSj 1vU07aD4olQmFJMlHFncSfHnFBFUZddMJgyxs5PuIiezj62SySgFxPrjzX4kprKH4tMALw1CpHfuj E0oBNAIlJbjBAfgFq0Pf+jmZD4CBAxzqyuAnft9BaMhVwCRTQYP5YVYT/nEm2jQHo9Dv8SCogafOu LdBM3fHdbJA5YSK/+uYMih9402exK8oHkprbbLoZATYd1CfT286VwSUnr0YqUSm0bigG6qRmIqYlx tgxJC2lLYrMBjBi1G88zbIjti3mWYEG15IYBq0b27IMkjXlrC3NJEktYCU2mxy9smDAGdYtvOJl83 ENZ5pQphA==; 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 (Red Hat Linux)) id 1i5QVd-0002GX-1w; Wed, 04 Sep 2019 08:19:21 +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 92453306027; Wed, 4 Sep 2019 10:18:42 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 139E429D882FB; Wed, 4 Sep 2019 10:19:19 +0200 (CEST) Date: Wed, 4 Sep 2019 10:19:19 +0200 From: Peter Zijlstra To: Joel Fernandes Cc: Viktor Rosendahl , paulmck@kernel.org, Steven Rostedt , Ingo Molnar , linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Subject: Re: [PATCH v5 1/4] ftrace: Implement fs notification for tracing_max_latency Message-ID: <20190904081919.GA2349@hirez.programming.kicks-ass.net> References: <20190903132602.3440-1-viktor.rosendahl@gmail.com> <20190903132602.3440-2-viktor.rosendahl@gmail.com> <20190904040039.GB150430@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190904040039.GB150430@google.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 Wed, Sep 04, 2019 at 12:00:39AM -0400, Joel Fernandes wrote: > [ Resending since I messed up my last email's headers! ] > > On Tue, Sep 03, 2019 at 03:25:59PM +0200, Viktor Rosendahl wrote: > > This patch implements the feature that the tracing_max_latency file, > > e.g. /sys/kernel/debug/tracing/tracing_max_latency will receive > > notifications through the fsnotify framework when a new latency is > > available. > > > > One particularly interesting use of this facility is when enabling > > threshold tracing, through /sys/kernel/debug/tracing/tracing_thresh, > > together with the preempt/irqsoff tracers. This makes it possible to > > implement a user space program that can, with equal probability, > > obtain traces of latencies that occur immediately after each other in > > spite of the fact that the preempt/irqsoff tracers operate in overwrite > > mode. > > Adding Paul since RCU faces similar situations, i.e. raising softirq risks > scheduler deadlock in rcu_read_unlock_special() -- but RCU's solution is to > avoid raising the softirq and instead use irq_work. Which is right. > I was wondering, if we can rename __raise_softirq_irqoff() to > raise_softirq_irqoff_no_wake() and call that from places where there is risk > of scheduler related deadlocks. Then I think this can be used from Viktor's > code. Let us discuss - what would happen if the softirq is raised, but > ksoftirqd is not awakened for this latency notification path? Is this really > an issue considering the softirq will execute during the next interrupt exit? You'd get unbounded latency for processing the softirq and warnings on going idle with softirqs pending. I really don't see why we should/want to be using softirq here.