From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932102AbeCLN1g (ORCPT ); Mon, 12 Mar 2018 09:27:36 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:49018 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751242AbeCLN1f (ORCPT ); Mon, 12 Mar 2018 09:27:35 -0400 Date: Mon, 12 Mar 2018 14:27:29 +0100 From: Peter Zijlstra To: Sebastian Andrzej Siewior Cc: Corey Minyard , Thomas Gleixner , Steven Rostedt , linux-rt-users , linux-kernel , Tejun Heo Subject: Re: Warning from swake_up_all in 4.14.15-rt13 non-RT Message-ID: <20180312132729.GI4064@hirez.programming.kicks-ass.net> References: <20180306174604.nta5rcvfvrfdfftz@linutronix.de> <1704d817-8fb9-ce8f-1aa1-fe6e8b0c3919@mvista.com> <20180308174103.mduy5qq2ttlcvig3@linutronix.de> <20180309110418.lwtennjqwqcxh422@linutronix.de> <20180309174605.GC4064@hirez.programming.kicks-ass.net> <20180309202550.j66qphz3txupt55u@linutronix.de> <20180309222643.GC5926@hirez.programming.kicks-ass.net> <20180312105113.p7jifhwdwbhpvxds@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180312105113.p7jifhwdwbhpvxds@linutronix.de> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 12, 2018 at 11:51:13AM +0100, Sebastian Andrzej Siewior wrote: > On 2018-03-09 23:26:43 [+0100], Peter Zijlstra wrote: > > On Fri, Mar 09, 2018 at 09:25:50PM +0100, Sebastian Andrzej Siewior wrote: > > > Is it just about the irqsave() usage or something else? I doubt it is > > > the list walk. It is still unbound if not called from irq-off region. > > > > The current list walk is preemptible. You put the entire iteration (of > > unbound length) inside a single critical section which destroy RT. > > I considered that list walk as cheap. We don't do any wake ups with the > list walk - just mark the task for a later wake up. But if it is not I > could add an upper limit of 20 iterations or so. So the problem is that as soon as this is exposed to userspace you've lost. If a user can stack like 10000 tasks on the completion before triggering it, you've got yourself a giant !preempt section. Yes the wake_q stuff is cheaper, but unbound is still unbound. wake_all must not be used from !preemptible (IRQ or otherwise) sections. And I'm not seeing how waking just the top 20 helps. > > Why isn't this a problem on RT? > So we remain in the preempt_disable() section due to RCU-sched so we > have this, yes. But the "disabled interrupts" part is due to > spin_lock_irqsave() which is a non-issue on RT. So if we managed to get > rid of the rcu-sched then the swait can go and we can stick with the > wake_up_all() on RT, too. OK, so for RT we simply loose the IRQ-disable thing, but its still a !preemptible section.