Re: [Qemu-devel] [RFC 0/6] monitor: allow per-monitor thread

[Fam Zheng <famz@redhat.com>]

On Mon, 08/21 16:36, Dr. David Alan Gilbert wrote:
> * Fam Zheng (famz@redhat.com) wrote:
> > On Mon, 08/21 18:05, Peter Xu wrote:
> > > On Mon, Aug 21, 2017 at 04:58:51PM +0800, Fam Zheng wrote:
> > > > On Mon, 08/21 15:44, Peter Xu wrote:
> > > > > This is an extended work for migration postcopy recovery. This series
> > > > > is tested with the following series to make sure it solves the monitor
> > > > > hang problem that we have encountered for postcopy recovery:
> > > > > 
> > > > >   [RFC 00/29] Migration: postcopy failure recovery
> > > > >   [RFC 0/6] migration: re-use migrate_incoming for postcopy recovery
> > > > > 
> > > > > The root problem is that, monitor commands are all handled in main
> > > > > loop thread now, no matter how many monitors we specify. And, if main
> > > > > loop thread hangs due to some reason, all monitors will be stuck.
> > > > > This can be done in reversed order as well: if any of the monitor
> > > > > hangs, it will hang the main loop, and the rest of the monitors (if
> > > > > there is any).
> > > > > 
> > > > > That affects postcopy recovery, since the recovery requires user input
> > > > > on destination side.  If monitors hang, the destination VM dies and
> > > > > lose hope for even a final recovery.
> > > > > 
> > > > > So, sometimes we need to make sure the monitor be alive, at least one
> > > > > of them.
> > > > > 
> > > > > The whole idea of this series is that instead if handling monitor
> > > > > commands all in main loop thread, we do it separately in per-monitor
> > > > > threads.  Then, even if main loop thread hangs at any point by any
> > > > > reason, per-monitor thread can still survive.  Further, we add hint in
> > > > > QMP/HMP to show whether a command can be executed without QMP, if so,
> > > > > we avoid taking BQL when running that command.  It greatly reduced
> > > > > contention of BQL.  Now the only user of that new parameter (currently
> > > > > I call it "without-bql") is "migrate-incoming" command, which is the
> > > > > only command to rescue a paused postcopy migration.
> > > > > 
> > > > > However, even with the series, it does not mean that per-monitor
> > > > > threads will never hang.  One example is that we can still run "info
> > > > > vcpus" in per-monitor threads during a paused postcopy (in that state,
> > > > > page faults are never handled, and "info cpus" will never return since
> > > > > it tries to sync every vcpus).  So to make sure it does not hang, we
> > > > > not only need the per-monitor thread, the user should be careful as
> > > > > well on how to use it.
> > > > 
> > > > I think this is like saying we expect the user to understand the internals of
> > > > QEMU, unless the "rules" are clearly documented.  Taking this into account,
> > > > does it make sense to make the per-monitor thread only allow BQL-free commands?
> > > 
> > > I don't think users need to know the internals - they just need to be
> > > careful on using them.  Just take the example of "info cpus": during
> > > paused postcopy it will hang, but IMHO it does not mean that it's
> > > illegal for user to send that command.  It's "by-design" that it'll be
> > > stuck if one of the vcpus is stuck somewhere; it's just not the
> > > correct way to use it when the monitor is prepared for postcopy
> > > recovery.
> > 
> > They still need to know "what" is the correct way to use the monitor, and what
> > I'm saying is there doesn't seem to be an easy way for users to know exactly
> > what is correct. See below.
> > 
> > > 
> > > And IMHO we should not treat threaded monitors special - it should be
> > > exactly the same monitor service when used with main loop thread.  It
> > > just has its own thread to handle the requests, so it is less
> > > dependent on main loop thread, and that's all.
> > 
> > It's not that simple, I think all non-trivial commands need very careful audit
> > before assuming they're safe. For example many block related commands
> > (qmp_trasaction, for example) indirectly calls BDRV_POLL_WHILE(), which, if
> > called from a per-monitor thread, will enter the else branch then fail the first
> > assert.
> 
> OK, that's interesting - I'd assumed that as long as we actually held
> the bql we were reasonably safe.
> Can you explain what that assert is actually asserting?

It's not much more than asserting qemu_mutex_iothread_locked(), the problem is
the new monitor thread breaks certain assumptions that was true.

What is interesting in this is that block layer's nested aio_poll() now not only
run in the main thread but also in the monitor thread. Bugs may hide there.  :)

That's why I suggested a "safe by default" strategy.


One step back, is it possible to "unblock" main thread even upon network issue?
What is the scenario that causes main thread hang? Is there a backtrace?

Fam