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Re: [PATCH v1 0/2] Add timeout mechanism to qmp actions

From: Stefan Hajnoczi
Subject: Re: [PATCH v1 0/2] Add timeout mechanism to qmp actions
Date: Mon, 14 Sep 2020 14:27:38 +0100

On Tue, Aug 11, 2020 at 09:54:08PM +0800, Zhenyu Ye wrote:
> Hi Kevin,
> On 2020/8/10 23:38, Kevin Wolf wrote:
> > Am 10.08.2020 um 16:52 hat Zhenyu Ye geschrieben:
> >> Before doing qmp actions, we need to lock the qemu_global_mutex,
> >> so the qmp actions should not take too long time.
> >>
> >> Unfortunately, some qmp actions need to acquire aio context and
> >> this may take a long time.  The vm will soft lockup if this time
> >> is too long.
> > 
> > Do you have a specific situation in mind where getting the lock of an
> > AioContext can take a long time? I know that the main thread can
> > block for considerable time, but QMP commands run in the main thread, so
> > this patch doesn't change anything for this case. It would be effective
> > if an iothread blocks, but shouldn't everything running in an iothread
> > be asynchronous and therefore keep the AioContext lock only for a short
> > time?
> > 
> Theoretically, everything running in an iothread is asynchronous. However,
> some 'asynchronous' actions are not non-blocking entirely, such as
> io_submit().  This will block while the iodepth is too big and I/O pressure
> is too high.  If we do some qmp actions, such as 'info block', at this time,
> may cause vm soft lockup.  This series can make these qmp actions safer.
> I constructed the scene as follow:
> 1. create a vm with 4 disks, using iothread.
> 2. add press to the CPU on the host.  In my scene, the CPU usage exceeds 95%.
> 3. add press to the 4 disks in the vm at the same time.  I used the fio and
> some parameters are:
>        fio -rw=randrw -bs=1M -size=1G -iodepth=512 -ioengine=libaio -numjobs=4
> 4. do block query actions, for example, by virsh:
>       virsh qemu-monitor-command [vm name] --hmp info block
> Then the vm will soft lockup, the calltrace is:
> [  192.311393] watchdog: BUG: soft lockup - CPU#1 stuck for 42s! 
> [kworker/1:1:33]

Sorry I haven't had time to investigate this myself yet.

Do you also have a QEMU backtrace when the hang occurs?

Let's find out if QEMU is stuck in the io_submit(2) syscall or whether
there's an issue in QEMU itself that causes the softlockup (for example,
aio_poll() with the global mutex held).


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