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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 66099C48BDF for ; Fri, 18 Jun 2021 19:00:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4BEBC611B0 for ; Fri, 18 Jun 2021 19:00:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234611AbhFRTCe (ORCPT ); Fri, 18 Jun 2021 15:02:34 -0400 Received: from mail.kernel.org ([198.145.29.99]:58032 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231589AbhFRTCd (ORCPT ); Fri, 18 Jun 2021 15:02:33 -0400 Received: from oasis.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 39E25610EA; Fri, 18 Jun 2021 19:00:22 +0000 (UTC) Date: Fri, 18 Jun 2021 15:00:20 -0400 From: Steven Rostedt To: Daniel Bristot de Oliveira Cc: Phil Auld , Sebastian Andrzej Siewior , Kate Carcia , Jonathan Corbet , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Alexandre Chartre , Clark Willaims , John Kacur , Juri Lelli , Borislav Petkov , "H. Peter Anvin" , x86@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH V4 05/12] trace/hwlat: Support hotplug operations Message-ID: <20210618150020.689439d4@oasis.local.home> In-Reply-To: <20210618124503.388fe4d4@oasis.local.home> References: <8899f8a8bec38bc600f7a2c61bc6ca664aa7beeb.1623746916.git.bristot@redhat.com> <20210618124503.388fe4d4@oasis.local.home> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 18 Jun 2021 12:45:03 -0400 Steven Rostedt wrote: > > +/* > > + * hwlat_cpu_init - CPU hotplug online callback function > > + */ > > +static int hwlat_cpu_init(unsigned int cpu) > > +{ > > + struct trace_array *tr = hwlat_trace; > > + > > You need to take the trace_types_lock here, between testing the > hwlat_busy and starting the threads. Otherwise, between the two, the > hwlat tracer could be turned off while a CPU is coming on line, and > then you just started a per cpu thread, while the hwlat tracer is not > enabled. And of course, because get_online_cpus() is called within trace_types_lock, doing this check is going to cause a lock inversion. The only thing I could think of is to wake up a worker thread to do the work. That is, this just wakes the worker thread, then the worker grabs the trace_types_lock, iterates through the cpu mask of expect running threads, and then starts or kills them depending on the hwlat_busy value. -- Steve > > > + if (!hwlat_busy) > > + return 0; > > + > > + if (!cpumask_test_cpu(cpu, tr->tracing_cpumask)) > > + return 0; > > + > > + return start_cpu_kthread(cpu); > > +}