From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sebastian Andrzej Siewior Subject: Re: Warning from swake_up_all in 4.14.15-rt13 non-RT Date: Mon, 12 Mar 2018 15:11:07 +0100 Message-ID: <20180312141107.roveviectc6a75s7@linutronix.de> 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> <20180312132729.GI4064@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: Corey Minyard , Thomas Gleixner , Steven Rostedt , linux-rt-users , linux-kernel , Tejun Heo To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <20180312132729.GI4064@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org On 2018-03-12 14:27:29 [+0100], Peter Zijlstra wrote: > 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. I know. We had this very same problem with clock_nanosleep() which got solved after timer rework. > 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. I assumed you complained about the unbounded list-walk with interrupts disabled (which is cheap but unbound is unbound). So here I suggested I move 20 entries off that list a time and enable interrupts again so an interrupt could set need_resched. But if we get invoked !preemptible then nothing changes. > > > 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. exactly. The irqsafe() was to guard non-RT config which uses the same code. So do I understand you correctly that irqsafe may remain for !RT config but that invocation with disabled preemption due to sched_rcu (on RT, too) must go? Sebastian