Re: [PATCH 2/6] migration: Kick postcopy threads on cancel

[Daniel P . Berrangé]

On Wed, Dec 04, 2024 at 03:51:27PM -0500, Peter Xu wrote:
> On Wed, Dec 04, 2024 at 08:02:31PM +0000, Daniel P. Berrangé wrote:
> > I would say the difference is like a graceful shutdown vs pulling the
> > power plug in a bare metal machine
> > 
> > 'cancel' is intended to be graceful. It should leave you with a functional
> > QEMU (or refuse to run if unsafe).
> > 
> > 'yank' is intended to be forceful, letting you get out of bad situations
> > that would otherwise require you to kill the entire QEMU process, but
> > still with possible associated risk data loss to the QEMU backends.
> > 
> > They have overlap, but are none the less different.
> 
> The question is more about whether yank should be used at all for
> migration only, not about the rest instances.
> 
> My answer is yank should never be used for migration, because
> "migrate_cancel" also unplugs the power plug.. It's not anything more
> enforced.  It's only doing less always.
> 
> E.g. migration_yank_iochannel() is exactly what we do with
> qmp_migrate_cancel() in the first place, only that migrate_cancel only does
> it on the main channel (on both qemufiles even if ioc is one), however it
> should be suffice, and behave the same way, as strong as "yank".

I recall at the time the yank stuff was introduced, one of the scenarios
they were concerned about was related to locks held by QEMU code. eg that
there are scenarios where migrate_cancel may not be processed promptly
enough due to being stalled on mutexes held by other concurrently running
threads. Now I would expect any such long duration stalls on migration
mutexes to be bugs, but the intent of yank is to give a recovery mechanism
that can workaround such bugs.  The yank QMP command only interacts with
its own local mutexes.

So even though both migrate-cancel and yank end up calling the same
qio_channel_shutdown() API, there can be practical differences in how
they reach that qio_channel_shutdown() call.


With regards,
Daniel
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