From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754118AbYIWUCv (ORCPT ); Tue, 23 Sep 2008 16:02:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752112AbYIWUCn (ORCPT ); Tue, 23 Sep 2008 16:02:43 -0400 Received: from gw.goop.org ([64.81.55.164]:58249 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751732AbYIWUCm (ORCPT ); Tue, 23 Sep 2008 16:02:42 -0400 Message-ID: <48D94B64.3070004@goop.org> Date: Tue, 23 Sep 2008 13:02:44 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.16 (X11/20080723) MIME-Version: 1.0 To: Ingo Molnar , "Eric W. Biederman" , Thomas Gleixner CC: Linux Kernel Mailing List Subject: Should irq_chip->mask disable percpu interrupts to all cpus, or just to this cpu? X-Enigmail-Version: 0.95.7 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, I'm reworking Xen's interrupt handling to isolate it a bit from the workings of the apic-based code, as Eric suggested a while back. As I've mentioned before, Xen represents interrupts as event channels. There are two major classes of event channels: per-cpu and, erm, not percpu. Per-cpu event channels are for things like timers and IPI function calls which are inherently per-cpu; it's meaningless to consider, for example, migrating them from cpu to cpu. I guess they're analogous to the the local apic vectors. (Non-percpu event channels can be bound to a particular cpu, and rebound at will; I'm not worried about them here.) Previously I allocated an irq per percpu event channel per cpu. This was pretty wasteful, since I need about 5-6 of them per cpu, so the number of interrupts increases quite quickly as cpus does. There's no deep problem with that, but it gets fairly ugly in /proc/interrupts, and there's some tricky corners to manage in suspend/resume. This time around I'm allocating a single irq for each percpu interrupt source (so one for timers, one for IPI, etc), and mapping each per-cpu event channel to each. But I'm wondering what the correct behaviour of irq_chip->mask/unmask should be in this case. Each event channel is individually maskable, so when ->mask gets called, I can either mask all the event channels associated with that irq, or just the one for this cpu. The latter makes most sense for me, but I don't quite understand the irq infrastructure enough to know if it will have bad effects globally. When I request the irq, I pass IRQF_PERCPU in the flags, but aside from preventing migration, this only seems to have an effect on __do_IRQ(), which looks like a legacy path anyway. It seems to me that by setting it that I'm giving the interrupt subsystem fair warning that ->mask() is only going to disable the interrupt on this cpu. Are there any other ill-effects of sharing an irq across all cpus like this? I guess there's some risk of contention on the irq_desc lock. Thanks, J