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 7445AC3A59F for ; Mon, 26 Aug 2019 17:49:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 51FC02184D for ; Mon, 26 Aug 2019 17:49:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387499AbfHZRt2 (ORCPT ); Mon, 26 Aug 2019 13:49:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54002 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728280AbfHZRt2 (ORCPT ); Mon, 26 Aug 2019 13:49:28 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BCEB58AE40; Mon, 26 Aug 2019 17:49:27 +0000 (UTC) Received: from ovpn-116-73.phx2.redhat.com (ovpn-116-73.phx2.redhat.com [10.3.116.73]) by smtp.corp.redhat.com (Postfix) with ESMTP id 98C71196AE; Mon, 26 Aug 2019 17:49:22 +0000 (UTC) Message-ID: <72c2da8695f622b8962ac43e3571107382969555.camel@redhat.com> Subject: Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep From: Scott Wood To: paulmck@linux.ibm.com, Sebastian Andrzej Siewior Cc: Joel Fernandes , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Juri Lelli , Clark Williams Date: Mon, 26 Aug 2019 12:49:22 -0500 In-Reply-To: <20190826162945.GE28441@linux.ibm.com> References: <20190821231906.4224-1-swood@redhat.com> <20190821231906.4224-3-swood@redhat.com> <20190823162024.47t7br6ecfclzgkw@linutronix.de> <433936e4c720e6b81f9b297fefaa592fd8a961ad.camel@redhat.com> <20190824031014.GB2731@google.com> <20190826152523.dcjbsgyyir4zjdol@linutronix.de> <20190826162945.GE28441@linux.ibm.com> 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: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Mon, 26 Aug 2019 17:49:27 +0000 (UTC) Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On Mon, 2019-08-26 at 09:29 -0700, Paul E. McKenney wrote: > On Mon, Aug 26, 2019 at 05:25:23PM +0200, Sebastian Andrzej Siewior wrote: > > On 2019-08-23 23:10:14 [-0400], Joel Fernandes wrote: > > > On Fri, Aug 23, 2019 at 02:28:46PM -0500, Scott Wood wrote: > > > > On Fri, 2019-08-23 at 18:20 +0200, Sebastian Andrzej Siewior wrote: > > > > > this looks like an ugly hack. This sleeping_lock_inc() is used > > > > > where we > > > > > actually hold a sleeping lock and schedule() which is okay. But > > > > > this > > > > > would mean we hold a RCU lock and schedule() anyway. Is that okay? > > > > > > > > Perhaps the name should be changed, but the concept is the same -- > > > > RT- > > > > specific sleeping which should be considered involuntary for the > > > > purpose of > > > > debug checks. Voluntary sleeping is not allowed in an RCU critical > > > > section > > > > because it will break the critical section on certain flavors of > > > > RCU, but > > > > that doesn't apply to the flavor used on RT. Sleeping for a long > > > > time in an > > > > RCU critical section would also be a bad thing, but that also > > > > doesn't apply > > > > here. > > > > > > I think the name should definitely be changed. At best, it is super > > > confusing to > > > call it "sleeping_lock" for this scenario. In fact here, you are not > > > even > > > blocking on a lock. > > > > > > Maybe "sleeping_allowed" or some such. > > > > The mechanism that is used here may change in future. I just wanted to > > make sure that from RCU's side it is okay to schedule here. > > Good point. > > The effect from RCU's viewpoint will be to split any non-rcu_read_lock() > RCU read-side critical section at this point. This alrady happens in a > few places, for example, rcu_note_context_switch() constitutes an RCU > quiescent state despite being invoked with interrupts disabled (as is > required!). The __schedule() function just needs to understand (and does > understand) that the RCU read-side critical section that would otherwise > span that call to rcu_node_context_switch() is split in two by that call. > > However, if this was instead an rcu_read_lock() critical section within > a PREEMPT=y kernel, then if a schedule() occured within stop_one_task(), > RCU would consider that critical section to be preempted. This means > that any RCU grace period that is blocked by this RCU read-side critical > section would remain blocked until stop_one_cpu() resumed, returned, > and so on until the matching rcu_read_unlock() was reached. In other > words, RCU would consider that RCU read-side critical section to span > the call to stop_one_cpu() even if stop_one_cpu() invoked schedule(). > > On the other hand, within a PREEMPT=n kernel, the call to schedule() > would split even an rcu_read_lock() critical section. Which is why I > asked earlier if sleeping_lock_inc() and sleeping_lock_dec() are no-ops > in !PREEMPT_RT_BASE kernels. We would after all want the usual lockdep > complaints in that case. migrate_enable() is PREEMPT_RT_BASE-specific -- this code won't execute at all with PREEMPT=n. -Scott