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

[Fam Zheng]

On 2020-09-19 10:22, Zhenyu Ye wrote:
> On 2020/9/18 22:06, Fam Zheng wrote:
> > 
> > I can see how blocking in a slow io_submit can cause trouble for main
> > thread. I think one way to fix it (until it's made truly async in new
> > kernels) is moving the io_submit call to thread pool, and wrapped in a
> > coroutine, perhaps.
> >
> 
> I'm not sure if any other operation will block the main thread, other
> than io_submit().

Then that's a problem with io_submit which should be fixed. Or more
precisely, that is a long held lock that we should avoid in QEMU's event
loops.

> 
> > I'm not sure qmp timeout is a complete solution because we would still
> > suffer from a blocked state for a period, in this exact situation before
> > the timeout.
> 
> Anyway, the qmp timeout may be the last measure to prevent the VM
> soft lockup. 

Maybe, but I don't think baking such a workaround into the QMP API is a
good idea. No QMP command should be synchronously long running, so
having a timeout parameter is just a wrong design.

Thanks,

Fam