On Tue, Mar 20, 2012 at 11:44 AM, Stefan Hajnoczi <stefa...@linux.vnet.ibm.com> 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? Stefan