Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758762AbZFQRuw (ORCPT ); Wed, 17 Jun 2009 13:50:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752495AbZFQRup (ORCPT ); Wed, 17 Jun 2009 13:50:45 -0400 Received: from x35.xmailserver.org ([64.71.152.41]:36252 "EHLO x35.xmailserver.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751355AbZFQRuo (ORCPT ); Wed, 17 Jun 2009 13:50:44 -0400 X-AuthUser: davidel@xmailserver.org Date: Wed, 17 Jun 2009 10:44:34 -0700 (PDT) From: Davide Libenzi X-X-Sender: davide@makko.or.mcafeemobile.com To: Gregory Haskins cc: "Michael S. Tsirkin" , kvm@vger.kernel.org, Linux Kernel Mailing List , avi@redhat.com, paulmck@linux.vnet.ibm.com, Ingo Molnar Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface In-Reply-To: <4A3927C0.5060607@novell.com> Message-ID: References: <20090616022041.23890.90120.stgit@dev.haskins.net> <20090616022956.23890.63776.stgit@dev.haskins.net> <20090616140240.GA9401@redhat.com> <4A37A7FC.4090403@novell.com> <20090616143816.GA18196@redhat.com> <4A37B0BB.3020005@novell.com> <20090616145502.GA1102@redhat.com> <4A37B832.6040206@novell.com> <20090616154150.GA17494@redhat.com> <4A37C592.2030407@novell.com> <4A37CFDA.4000602@novell.com> <4A3927C0.5060607@novell.com> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) X-GPG-FINGRPRINT: CFAE 5BEE FD36 F65E E640 56FE 0974 BF23 270F 474E X-GPG-PUBLIC_KEY: http://www.xmailserver.org/davidel.asc MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2108 Lines: 48 On Wed, 17 Jun 2009, Gregory Haskins wrote: > Can you elaborate? I currently do not see how I could do the proposed > concept inside of irqfd while still using eventfd. Of course, that > would be possible if we fork irqfd from eventfd, and perhaps this is > what you are proposing. As previously stated I don't want to give up on > the prospect of re-using it quite yet, so bear with me. :) > > The issue with eventfd, as I see it, is that eventfd uses a > spin_lock_irqsave (by virtue of the wait-queue stuff) across the > "signal" callback (which today is implemented as a wake-up). This > spin_lock implicitly creates a non-preemptible critical section that > occurs independently of whether eventfd_signal() itself is invoked from > a sleepable context or not. > > What I strive to achieve is to remove the creation of this internal > critical section. If eventfd_signal() is called from atomic context, so > be it. We will detect this in the callback and be forced to take the > slow-path, and I am ok with that. *But*, if eventfd_signal() (or > f_ops->write(), for that matter) are called from a sleepable context > *and* eventfd doesn't introduce its own critical section (such as with > my srcu patch), we can potentially optimize within the callback by > executing serially instead of deferring (e.g. via a workqueue). Since when the scheduling (assuming it's not permanently running on another core due to high frequency work post) of a kernel thread is such a big impact that interfaces need to be redesigned for that? How much the (possible, but not certain) kernel thread context switch time weighs in the overall KVM IRQ service time? > It can! :) This is not changing from whats in mainline today (covered > above). It can/could, if the signal() function takes very accurate care of doing the magic atomic check. - Davide -- 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/