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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 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 853D2C352AA for ; Wed, 2 Oct 2019 15:13:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 611F421920 for ; Wed, 2 Oct 2019 15:13:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728464AbfJBPN3 (ORCPT ); Wed, 2 Oct 2019 11:13:29 -0400 Received: from mail.kernel.org ([198.145.29.99]:60036 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728076AbfJBPN1 (ORCPT ); Wed, 2 Oct 2019 11:13:27 -0400 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id A7AB121920; Wed, 2 Oct 2019 15:13:26 +0000 (UTC) Date: Wed, 2 Oct 2019 11:13:24 -0400 From: Steven Rostedt To: "Viktor Rosendahl (BMW)" Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Joel Fernandes Subject: Re: [PATCH v7 1/4] ftrace: Implement fs notification for tracing_max_latency Message-ID: <20191002111324.7590bf6d@gandalf.local.home> In-Reply-To: <20190920152219.12920-2-viktor.rosendahl@gmail.com> References: <20190920152219.12920-1-viktor.rosendahl@gmail.com> <20190920152219.12920-2-viktor.rosendahl@gmail.com> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 20 Sep 2019 17:22:16 +0200 "Viktor Rosendahl (BMW)" 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. > > This facility works with the hwlat, preempt/irqsoff, and wakeup > tracers. > > The tracers may call the latency_fsnotify() from places such as > __schedule() or do_idle(); this makes it impossible to call > queue_work() directly without risking a deadlock. The same would > happen with a softirq, kernel thread or tasklet. For this reason we > use the irq_work mechanism to call queue_work(). Can fsnotify() be called from irq context? If so, why have the work queue at all? Just do the work from the irq_work handler. -- Steve > > This patch creates a new workqueue. The reason for doing this is that > I wanted to use the WQ_UNBOUND and WQ_HIGHPRI flags. My thinking was > that WQ_UNBOUND might help with the latency in some important cases. > > If we use: > > queue_work(system_highpri_wq, &tr->fsnotify_work); > > then the work will (almost) always execute on the same CPU but if we are > unlucky that CPU could be too busy while there could be another CPU in > the system that would be able to process the work soon enough. > > queue_work_on() could be used to queue the work on another CPU but it > seems difficult to select the right CPU. >