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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 30E92C432C1 for ; Tue, 24 Sep 2019 15:47:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 030F521655 for ; Tue, 24 Sep 2019 15:47:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388230AbfIXPrl (ORCPT ); Tue, 24 Sep 2019 11:47:41 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39518 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388100AbfIXPrl (ORCPT ); Tue, 24 Sep 2019 11:47:41 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BE33E1DA3; Tue, 24 Sep 2019 15:47:40 +0000 (UTC) Received: from ovpn-117-172.phx2.redhat.com (ovpn-117-172.phx2.redhat.com [10.3.117.172]) by smtp.corp.redhat.com (Postfix) with ESMTP id 659B760BFB; Tue, 24 Sep 2019 15:47:37 +0000 (UTC) Message-ID: <1a2234884e55e5ee6df5f32f828a99c1b248933f.camel@redhat.com> Subject: Re: [PATCH RT v3 3/5] sched: migrate_dis/enable: Use rt_invol_sleep From: Scott Wood To: Sebastian Andrzej Siewior Cc: linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, "Paul E . McKenney" , Joel Fernandes , Thomas Gleixner , Steven Rostedt , Peter Zijlstra , Juri Lelli , Clark Williams Date: Tue, 24 Sep 2019 10:47:36 -0500 In-Reply-To: <20190924152514.enzeuoo5a6o3mgqu@linutronix.de> References: <20190911165729.11178-1-swood@redhat.com> <20190911165729.11178-4-swood@redhat.com> <20190917075943.qsaakyent4dxjkq4@linutronix.de> <779eddcc937941e65659a11b1867c6623a2c8890.camel@redhat.com> <404575720cf24765e66020f15ce75352f08a0ddb.camel@redhat.com> <20190923175233.yub32stn3xcwkaml@linutronix.de> <20190924112155.rxeyksetgqmer3pg@linutronix.de> <55dc19fcc44b2e658b71f68206306c8310335564.camel@redhat.com> <20190924152514.enzeuoo5a6o3mgqu@linutronix.de> Organization: Red Hat Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.30.5 (3.30.5-1.fc29) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.71]); Tue, 24 Sep 2019 15:47:41 +0000 (UTC) Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On Tue, 2019-09-24 at 17:25 +0200, Sebastian Andrzej Siewior wrote: > On 2019-09-24 08:53:43 [-0500], Scott Wood wrote: > > As I pointed out in the "[PATCH RT 6/8] sched: migrate_enable: Set state > > to > > TASK_RUNNING" discussion, we can get here inside the rtmutex code (e.g. > > from > > debug_rt_mutex_print_deadlock) where saved_state is already holding > > something -- plus, the waker won't have WF_LOCK_SLEEPER and therefore > > saved_state will get cleared anyway. > > So let me drop the saved_state pieces and get back to it once I get to > the other thread (which you replied and I didn't realised until now). > > Regarding the WF_LOCK_SLEEPER part. I think this works as expected. > Imagine: > > CPU0 CPU1 > spin_lock(); > set_current_state(TASK_UNINTERRUPTIBLE); > … > spin_unlock() > -> migrate_enable(); > -> stop_one_cpu(); <-- A) > other_func(); <-- B) > schedule(); > > So. With only CPU0 we enter schedule() with TASK_UNINTERRUPTIBLE because > the state gets preserved with the change I added (which is expected). > If CPU1 sends a wake_up() at A) then the saved_state gets overwritten > and we enter schedule() with TASK_RUNNING. Same happens if it is sent at > B) point which is outside of any migrate/spin lock related code. > > Was this clear or did I miss the point? When the stop machine finishes it will do a wake_up_process() via complete(). Since this does not pass WF_LOCK_SLEEPER, saved_state will be cleared, and you'll have TASK_RUNNING when you get to other_func() and schedule(), regardless of whether CPU1 sends wake_up() -- so this change doesn't actually accomplish anything. While as noted in the other thread I don't think these spurious wakeups are a huge problem, we could avoid them by doing stop_one_cpu_nowait() and then schedule() without messing with task state. Since we're stopping our own cpu, it should be guaranteed that the stopper has finished by the time we exit schedule(). -Scott