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

[Daniel P . Berrangé]

On Thu, Dec 05, 2024 at 10:03:58AM -0500, Peter Xu wrote:
> On Thu, Dec 05, 2024 at 10:18:53AM -0300, Fabiano Rosas wrote:
> > Daniel P. Berrangé <berrange@redhat.com> writes:
> > 
> > > 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.
> > 
> > Ok, so that could only mean a thread stuck in recv() while holding the
> > BQL. I don't think we have any other locks which would stop
> > migrate_cancel from making progress or other stall situations that could
> > be helped by a shutdown(). Note that most of locks around qemu_file were
> > a late addition. I don't think that scenario is possible today. I'll
> > have to do some tests.
> 
> And if that is a real difference, I'd think whether we should simply make
> migrate_cancel be oob-capable too.. IOW, I still think it'll be good to
> stick with always one API to cancel a migration, no matter which it is.  If
> we want to move over to yank then I think we should move all migrate_cancel
> operations into yank and deprecate "migrate_cancel', but that sounds
> overkill.

Well migrate_cancel ought to be safer than yank. eg migrate_cancel (sh|c)ould
refuse to run if issued during post-copy phase. Or even in precopy, if
in the final vmstate copy & switchover phase we shouldn't need to cancel.
yank meanwhile will always run, no matter what, because by design, it has
no interaction with the migration code beyond knowing that a socket exists.

I don't  think we should combine them. They have alot of common, but there
are subtle differences that are relevant to the scenarios in which thye
are intended to be used.

> There's only one thing that might not be oob-compatible there so far, which
> is bdrv_activate_all().  But I plan to remove it very soon (so that disks
> will be activated in the migration thread instead, just like failure cases).
> 
> > 
> > On that note, how is yank supposed to be accessed? I don't see support
> > in libvirt. Is there a way to hook into QMP after the fact somehow?
> > 
> 
> -- 
> Peter Xu
> 

With regards,
Daniel
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