From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: Role of qemu_fair_mutex Date: Mon, 03 Jan 2011 12:08:27 +0200 Message-ID: <4D21A01B.7000900@redhat.com> References: <4D219AF5.2030204@web.de> <4D219E6D.8060902@redhat.com> <4D219F06.7040305@web.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Cc: qemu-devel , kvm , Marcelo Tosatti To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:43897 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751426Ab1ACKId (ORCPT ); Mon, 3 Jan 2011 05:08:33 -0500 In-Reply-To: <4D219F06.7040305@web.de> Sender: kvm-owner@vger.kernel.org List-ID: On 01/03/2011 12:03 PM, Jan Kiszka wrote: > Am 03.01.2011 11:01, Avi Kivity wrote: > > On 01/03/2011 11:46 AM, Jan Kiszka wrote: > >> Hi, > >> > >> at least in kvm mode, the qemu_fair_mutex seems to have lost its > >> function of balancing qemu_global_mutex access between the io-thread and > >> vcpus. It's now only taken by the latter, isn't it? > >> > >> This and the fact that qemu-kvm does not use this kind of lock made me > >> wonder what its role is and if it is still relevant in practice. I'd > >> like to unify the execution models of qemu-kvm and qemu, and this lock > >> is the most obvious difference (there are surely more subtle ones as > >> well...). > >> > > > > IIRC it was used for tcg, which has a problem that kvm doesn't have: a > > tcg vcpu needs to hold qemu_mutex when it runs, which means there will > > always be contention on qemu_mutex. In the absence of fairness, the tcg > > thread could dominate qemu_mutex and starve the iothread. > > > > This doesn't happen with kvm since kvm vcpus drop qemu_mutex when running. > > > > I see. Then I guess we should do this: > > diff --git a/cpus.c b/cpus.c > index 9bf5224..0de8552 100644 > --- a/cpus.c > +++ b/cpus.c > @@ -734,9 +734,7 @@ static sigset_t block_io_signals(void) > void qemu_mutex_lock_iothread(void) > { > if (kvm_enabled()) { > - qemu_mutex_lock(&qemu_fair_mutex); > qemu_mutex_lock(&qemu_global_mutex); > - qemu_mutex_unlock(&qemu_fair_mutex); > } else { > qemu_mutex_lock(&qemu_fair_mutex); > if (qemu_mutex_trylock(&qemu_global_mutex)) { I think so, though Anthony or Marcelo should confirm my interpretation first. -- error compiling committee.c: too many arguments to function