From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:60917) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SCAu7-0001wE-Ih for qemu-devel@nongnu.org; Mon, 26 Mar 2012 10:28:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1SCAu1-0000Pb-Vj for qemu-devel@nongnu.org; Mon, 26 Mar 2012 10:28:15 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40913) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SCAu1-0000P4-Mz for qemu-devel@nongnu.org; Mon, 26 Mar 2012 10:28:09 -0400 Message-ID: <4F707DCD.2010000@redhat.com> Date: Mon, 26 Mar 2012 16:31:41 +0200 From: Kevin Wolf MIME-Version: 1.0 References: <1331603611-9103-1-git-send-email-zwu.kernel@gmail.com> <4F685230.9040209@redhat.com> <4F6854B2.8000209@redhat.com> <20120320114442.GA30819@stefanha-thinkpad.localdomain> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit 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: Stefan Hajnoczi Cc: Zhi Yong Wu , Paolo Bonzini , Zhi Yong Wu , qemu-devel@nongnu.org, Stefan Hajnoczi Am 26.03.2012 16:21, schrieb 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 QEMU >>>>> 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 save. >>>> As 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. See 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. Throttling itself is not the problem. >> We've known that synchronous operations in the vcpu thread are a problem >> long before throttling. This 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. But 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) { >> fprintf(stderr, "Disabling I/O throttling on '%s' due " >> "to synchronous I/O\n", bdrv_get_device_name(bs)); >> bdrv_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? No, I don't have any clever short-term solution to offer. Put in whatever hack you think works best for 1.1. Kevin