Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756143Ab2FNPCP (ORCPT ); Thu, 14 Jun 2012 11:02:15 -0400 Received: from www.linutronix.de ([62.245.132.108]:58309 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756054Ab2FNPCO (ORCPT ); Thu, 14 Jun 2012 11:02:14 -0400 Date: Thu, 14 Jun 2012 17:02:11 +0200 (CEST) From: Thomas Gleixner To: "Paul E. McKenney" cc: LKML , Peter Zijlstra , Ingo Molnar , "Srivatsa S. Bhat" , Rusty Russell , Tejun Heo Subject: Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu hotplug threads In-Reply-To: <20120614145038.GB2458@linux.vnet.ibm.com> Message-ID: References: <20120613102823.373180763@linutronix.de> <20120613105815.206105518@linutronix.de> <20120613185748.GF2427@linux.vnet.ibm.com> <20120613191745.GG2427@linux.vnet.ibm.com> <20120613204725.GA9858@linux.vnet.ibm.com> <20120614045125.GA30257@linux.vnet.ibm.com> <20120614125939.GC2443@linux.vnet.ibm.com> <20120614145038.GB2458@linux.vnet.ibm.com> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2542 Lines: 69 On Thu, 14 Jun 2012, Paul E. McKenney wrote: > On Thu, Jun 14, 2012 at 03:32:19PM +0200, Thomas Gleixner wrote: > > On Thu, 14 Jun 2012, Paul E. McKenney wrote: > > > On Thu, Jun 14, 2012 at 01:20:39PM +0200, Thomas Gleixner wrote: > > > > I gave it a quick shot, but I was not able to reproduce the hang yet. > > > > > > Really? I have a strictly Western-Hemisphere bug? ;-) > > > > I guess I need to fire up rcu torture to make it surface. > > A simple offline was triggering it for me. Perhaps some of my debug > code was inappropriate, will retry. > > > > > But looking at the thread function made me look into rcu_yield() and I > > > > really wonder what kind of drug induced that particular piece of > > > > horror. > > > > > > When you are working on something like RCU priority boosting, no other > > > drug is in any way necessary. ;-) > > > > And how do we protect minors from that ? > > We rely on their own sense of self-preservation preventing them from > getting involved in such insanity. > > > > > I can't figure out why this yield business is necessary at all. The > > > > commit logs are as helpful as the missing code comments :) > > > > > > > > I suspect that it's some starvation issue. But if we need it, then > > > > can't we replace it with something sane like the (untested) patch > > > > below? > > > > > > Yep, starvation. I will take a look at your approach after I wake > > > up a bit more. > > > > Btw, if that simpler yield approach is working and I can't see why it > > shouldn't then you can get rid of the node task as well. The only > > purpose of it is to push up the priority of yielding tasks, right? > > It also boosts the priority of preempted RCU read-side critical sections. Though the only way how this thread is invoked is via the timeout of that yield timer. So I really have a hard time for understanding that. cpu_kthread() .... yield() timer fires -> mark cpu in mask and wakeup node kthread node_kthread() do magic boost invocation push priority of cpu_kthread marked in mask For the boost thread it looks like: boost_kthread() ..... yield() timer fires -> wakeup node kthread node_kthread() do magic boost invocation, but no prio adjustment of boost thread. /me scratches head -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/