Re: [RFC PATCH 0/3] block: Synchronous bdrv_*() from coroutine in different AioContext

[Kevin Wolf]

Am 14.05.2020 um 15:21 hat Thomas Lamprecht geschrieben:
> On 5/12/20 4:43 PM, Kevin Wolf wrote:
> > Stefan (Reiter), after looking a bit closer at this, I think there is no
> > bug in QEMU, but the bug is in your coroutine code that calls block
> > layer functions without moving into the right AioContext first. I've
> > written this series anyway as it potentially makes the life of callers
> > easier and would probably make your buggy code correct.
> 
> > However, it doesn't feel right to commit something like patch 2 without
> > having a user for it. Is there a reason why you can't upstream your
> > async snapshot code?
> 
> I mean I understand what you mean, but it would make the interface IMO so
> much easier to use, if one wants to explicit schedule it beforehand they
> can still do. But that would open the way for two styles doing things, not
> sure if this would seen as bad. The assert about from patch 3/3 would be
> already really helping a lot, though.

I think patches 1 and 3 are good to be committed either way if people
think they are useful. They make sense without the async snapshot code.

My concern with the interface in patch 2 is both that it could give
people a false sense of security and that it would be tempting to write
inefficient code.

Usually, you won't have just a single call into the block layer for a
given block node, but you'll perform multiple operations. Switching to
the target context once rather than switching back and forth in every
operation is obviously more efficient.

But chances are that even if one of these function is bdrv_flush(),
which now works correctly from a different thread, you might need
another function that doesn't implement the same magic. So you always
need to be aware which functions support cross-context calls which
ones don't.

I feel we'd see a few bugs related to this.

> Regarding upstreaming, there was some historical attempt to upstream it
> from Dietmar, but in the time frame of ~ 8 to 10 years ago or so.
> I'm not quite sure why it didn't went through then, I see if I can get
> some time searching the mailing list archive.
> 
> We'd be naturally open and glad to upstream it, what it effectively
> allow us to do is to not block the VM to much during snapshoting it
> live.

Yes, there is no doubt that this is useful functionality. There has been
talk about this every now and then, but I don't think we ever got to a
point where it actually could be implemented.

Vladimir, I seem to remember you (or someone else from your team?) were
interested in async snapshots as well a while ago?

> I pushed a tree[0] with mostly just that specific code squashed together (hope
> I did not break anything), most of the actual code is in commit [1].
> It'd be cleaned up a bit and checked for coding style issues, but works good
> here.
> 
> Anyway, thanks for your help and pointers!
> 
> [0]: https://github.com/ThomasLamprecht/qemu/tree/savevm-async
> [1]: 
> https://github.com/ThomasLamprecht/qemu/commit/ffb9531f370ef0073e4b6f6021f4c47ccd702121

It doesn't even look that bad in terms of patch size. I had imagined it
a bit larger.

But it seems this is not really just an async 'savevm' (which would save
the VM state in a qcow2 file), but you store the state in a separate
raw file. What is the difference between this and regular migration into
a file?

I remember people talking about how snapshotting can store things in a
way that a normal migration stream can't do, like overwriting outdated
RAM state instead of just appending the new state, but you don't seem to
implement something like this.

Kevin