Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932788Ab3EOOrm (ORCPT ); Wed, 15 May 2013 10:47:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:27613 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932331Ab3EOOrk (ORCPT ); Wed, 15 May 2013 10:47:40 -0400 Date: Wed, 15 May 2013 11:06:53 -0400 From: Don Zickus To: Peter Zijlstra Cc: Frederic Weisbecker , Ingo Molnar , LKML , Steven Rostedt , "Paul E. McKenney" , Andrew Morton , Thomas Gleixner , "H. Peter Anvin" Subject: Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled Message-ID: <20130515150652.GP23604@redhat.com> References: <1368547372-21011-1-git-send-email-fweisbec@gmail.com> <1368547372-21011-2-git-send-email-fweisbec@gmail.com> <20130515083729.GC10510@laptop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130515083729.GC10510@laptop.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1716 Lines: 40 On Wed, May 15, 2013 at 10:37:29AM +0200, Peter Zijlstra wrote: > On Tue, May 14, 2013 at 06:02:51PM +0200, Frederic Weisbecker wrote: > > From: Steven Rostedt > > > > Trying to test the nohz_full code, I was not able to get it to work. > > Finally I enabled the tick_stop tracepoint and it showed: > > > > tick_stop: success=no msg=perf events running > > > > I talked to Frederic Weisbecker about this and he informed me that > > perf is used by the lockup detector. I checked the code, and sure > > enough it is. > > > > As perf is always running when LOCKUP_DETECTOR is enabled, which > > will always disable nohz_full from working, instead of confusing > > users, disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled. > > > > When perf is changed such that it does not prevent nohz_full from > > working, then we can and should remove this constraint. > > That's a bit contradictory in function, you want the NMI watchdog to > cover all code, so disabling whilst entering NO_HZ state is going to > make it not cover some code - *fail*. > > Rather I would suggest disabling the NMI watchdog's runtime default; so > you can still enable it with something like: > > echo 1 > /proc/sys/kernel/nmi_watchdog Coming into the middle of the thread is always hard, but why/how does perf disable nohz_full? I didn't think the hardware events of perf would cause problems as they are no different than an irq. Curious. Cheers, Don -- 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/