From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753784AbbJ0J3g (ORCPT ); Tue, 27 Oct 2015 05:29:36 -0400 Received: from mail-pa0-f53.google.com ([209.85.220.53]:33457 "EHLO mail-pa0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752492AbbJ0J3e (ORCPT ); Tue, 27 Oct 2015 05:29:34 -0400 Subject: Re: [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ To: Yunhong Jiang , Alex Williamson References: <1445908801-14732-1-git-send-email-yunhong.jiang@linux.intel.com> <1445917034.8018.220.camel@redhat.com> <20151027063501.GA22054@jnakajim-build> Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org From: Paolo Bonzini Message-ID: <562F43F8.1040101@redhat.com> Date: Tue, 27 Oct 2015 10:29:28 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <20151027063501.GA22054@jnakajim-build> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27/10/2015 07:35, Yunhong Jiang wrote: > On Mon, Oct 26, 2015 at 09:37:14PM -0600, Alex Williamson wrote: >> On Mon, 2015-10-26 at 18:20 -0700, Yunhong Jiang wrote: >>> An option to force VFIO PCI MSI/MSI-X handler as non-threaded IRQ, >>> even when CONFIG_IRQ_FORCED_THREADING=y. This is uselful when >>> assigning a device to a guest with low latency requirement since it >>> reduce the context switch to/from the IRQ thread. >> >> Is there any way we can do this automatically? Perhaps detecting that >> we're on a RT kernel or maybe that the user is running with RT priority? >> I find that module options are mostly misunderstood and misused. > > Alex, thanks for review. > > It's not easy to detect if the user is running with RT priority, since > sometimes the user start the thread and then set the scheduler priority > late. > > Also should we do this only for in kernel irqchip scenario and not for user > space handler, since in kernel irqchip has lower overhead? The overhead of the non-threaded IRQ handler is the same for kernel or userspace irqchip, since the handler just writes 1 to the eventfd. On RT kernels however can you call eventfd_signal from interrupt context? You cannot call spin_lock_irqsave (which can sleep) from a non-threaded interrupt handler, can you? You would need a raw spin lock. Paolo