From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759117Ab1LOQ2k (ORCPT ); Thu, 15 Dec 2011 11:28:40 -0500 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:52845 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759064Ab1LOQ2j (ORCPT ); Thu, 15 Dec 2011 11:28:39 -0500 Date: Thu, 15 Dec 2011 16:28:24 +0000 From: Will Deacon To: "Eric W. Biederman" Cc: "tglx@linutronix.de" , "linux-kernel@vger.kernel.org" Subject: Re: IRQ migration on CPU offline path Message-ID: <20111215162824.GM15131@mudshark.cambridge.arm.com> References: <20111214183446.GB12703@mudshark.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Hi Eric, Cheers for the response. On Thu, Dec 15, 2011 at 01:25:19AM +0000, Eric W. Biederman wrote: > Will Deacon writes: > > I've been looking at the IRQ migration code on x86 (fixup_irqs) for the CPU > > hotplug path in order to try and fix a bug we have on ARM with the > > desc->affinity mask not getting updated. Compared to irq_set_affinity, the code > > is pretty whacky (I guess because it's called via stop_machine) so I wondered if > > you could help me understand a few points: > > There is a lot of craziness on that path because of poor hardware design > on x86 we can't know when an irq has actually be migrated, and other > nasties. > > There is also the issue that I expect is still the case that we have the > generic layer asking us to cpu migration and the associated irq > migrations with the irqs disabled which at least for the bits of poorly > designed hardware made the entire path a best effort beast. Argh, ok. Does this mean that other architectures should just preserve the interface that x86 gives (for example not triggering IRQ affinity notifiers)? > If x86 becomes a good clean example in this corner case I would be > amazed. Last I looked I almost marked it all as CONFIG_BROKEN because > we were trying to do the impossible. Unfortunately peoples laptops > go through this path when they suspend and so it was more painful to > disable hacky racy mess than to keep living with it. > > There has been an increase in the number of cases where it is possible > to actually perform the migration with irqs disabled so on a good day > that code might even work. Right, so this stuff is fairly fragile. We can probably get a reasonable version working on ARM (with the GIC present) but I'm not sure what to do about the notifiers I mentioned earlier and proper migration of threaded interrupt handlers. I'll take a look at some other archs. Thanks, Will