From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [RFC][PATCH 1/3] Introduce a workqueue to deliver PIT timer interrupts. Date: Wed, 9 Jun 2010 10:23:35 -0300 Message-ID: <20100609132335.GA7396@amt.cnet> References: <1276019703-18136-1-git-send-email-clalance@redhat.com> <1276019703-18136-2-git-send-email-clalance@redhat.com> <4C0EFE20.1080508@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Chris Lalancette , kvm@vger.kernel.org, avi@redhat.com To: Zachary Amsden Return-path: Received: from mx1.redhat.com ([209.132.183.28]:63776 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752585Ab0FINuQ (ORCPT ); Wed, 9 Jun 2010 09:50:16 -0400 Received: from int-mx05.intmail.prod.int.phx2.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.18]) by mx1.redhat.com (8.13.8/8.13.8) with ESMTP id o59DoEBk030129 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 9 Jun 2010 09:50:15 -0400 Content-Disposition: inline In-Reply-To: <4C0EFE20.1080508@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, Jun 08, 2010 at 04:36:16PM -1000, Zachary Amsden wrote: > >+ pt->timer.function = pit_timer_fn; > > > > I am happy to see this. I thought kvm_timer_fn was a step > backwards; it was too general of a function to justify the savings > of 20 some odd lines of code. This was not done to save 20 lines of code: http://www.mail-archive.com/kvm@vger.kernel.org/msg18640.html > Is there any chance that using a workqueue might help the problem of > hrtimers firing too quickly? I wanted to return HR_NORESTART from > pit_timer_fn always, then restart the hrtimer on delivery, but > because of unreliable delivery, it wasn't clear how to do that. > > Perhaps the workqueue can be used to restart the timer instead, > avoiding problems of impossibly small timeouts causing hrtimers to > run amok. It should be rearmed on ACK ideally. How come delivery is unreliable? BTW, a massive amount of hrtimers (_single_ non-tickless 32-vcpu guest with LAPIC) results in: hrtimer: interrupt took 122448 ns in the host. So don't even need impossibly small timeouts :(