From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:54469) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SCAoC-0005eb-Ug for qemu-devel@nongnu.org; Mon, 26 Mar 2012 10:22:13 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1SCAo6-0006aZ-I4 for qemu-devel@nongnu.org; Mon, 26 Mar 2012 10:22:08 -0400 Received: from mail-lpp01m010-f45.google.com ([209.85.215.45]:48507) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SCAo6-0006Vp-81 for qemu-devel@nongnu.org; Mon, 26 Mar 2012 10:22:02 -0400 Received: by lahe6 with SMTP id e6so4060415lah.4 for ; Mon, 26 Mar 2012 07:22:00 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20120320114442.GA30819@stefanha-thinkpad.localdomain> References: <1331603611-9103-1-git-send-email-zwu.kernel@gmail.com> <4F685230.9040209@redhat.com> <4F6854B2.8000209@redhat.com> <20120320114442.GA30819@stefanha-thinkpad.localdomain> Date: Mon, 26 Mar 2012 15:21:59 +0100 Message-ID: From: Stefan Hajnoczi Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH] block: add the support to drain throttled requests List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Zhi Yong Wu , Paolo Bonzini , Zhi Yong Wu , qemu-devel@nongnu.org, Stefan Hajnoczi On Tue, Mar 20, 2012 at 11:44 AM, Stefan Hajnoczi wrote: > On Tue, Mar 20, 2012 at 10:58:10AM +0100, Kevin Wolf wrote: >> Am 20.03.2012 10:47, schrieb Paolo Bonzini: >> > Il 20/03/2012 10:40, Zhi Yong Wu ha scritto: >> >> HI, Kevin, >> >> >> >> We hope that I/O throttling can be shipped without known issue in QEM= U >> >> 1.1, so if you are available, can you give this patch some love? >> > >> > I'm sorry to say this, but I think I/O throttling is impossible to sav= e. >> > =A0As it is implemented now, it just cannot work in the presence of >> > synchronous I/O, except at the cost of busy waiting with the global >> > mutex taken. =A0See the message from Stefan yesterday. >> >> qemu_aio_flush() is busy waiting with the global mutex taken anyway, so >> it doesn't change that much. > > Yesterday I only posted an analysis of the bug but here are some > thoughts on how to move forward. =A0Throttling itself is not the problem. > We've known that synchronous operations in the vcpu thread are a problem > long before throttling. =A0This is just another reason to convert device > emulation to use asynchronous interfaces. > > Here is the list of device models that perform synchronous block I/O: > hw/fdc.c > hw/ide/atapi.c > hw/ide/core.c > hw/nand.c > hw/onenand.c > hw/pflash_cfi01.c > hw/pflash_cfi02.c > hw/sd.c > > Zhi Hui Li is working on hw/fdc.c and recently sent a patch. > > I think it's too close to QEMU 1.1 to convert all the remaining devices > and test them properly before the soft-freeze. =A0But it's probably > possible to convert IDE before the soft-freeze. > > In the meantime we could add this to bdrv_rw_co(): > > if (bs->io_limits_enabled) { > =A0 =A0fprintf(stderr, "Disabling I/O throttling on '%s' due " > =A0 =A0 =A0 =A0 =A0 =A0"to synchronous I/O\n", bdrv_get_device_name(bs)); > =A0 =A0bdrv_io_limits_disable(bs); > } > > It's not pretty but tells the user there is an issue and avoids > deadlocking. No one has commented on this suggestion. I think leaving a known hang in QEMU 1.1 is undesirable. Better to have this warning and disable throttling in the case we cannot support right now. Kevin: Would you accept a patch like this? Or do you have another solution in mind? Stefan