Re: [PATCH RFC C0/2] support allocation-map for block-dirty-bitmap-merge

[Markus Armbruster <armbru@redhat.com>]

John Snow <jsnow@redhat.com> writes:

> On 4/27/21 7:11 AM, Vladimir Sementsov-Ogievskiy wrote:
>> Hi all!
>> It's a simpler alternative for
>> "[PATCH v4 0/5] block: add block-dirty-bitmap-populate job"
>>    <20200902181831.2570048-1-eblake@redhat.com>
>>    https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg00978.html
>>    https://patchew.org/QEMU/20200902181831.2570048-1-eblake@redhat.com/
>> Since we have "coroutine: true" feature for qmp commands, I think,
>> maybe we can merge allocation status to bitmap without bothering with
>> new block-job?
>> It's an RFC:
>> 1. Main question: is it OK as a simple blocking command, even in a
>> coroutine mode. It's a lot simpler, and it can be simply used in a
>> transaction with other bitmap commands.
>> 
>
> Hm, possibly... I did not follow the discussion of coroutine QMP
> commands closely to know what the qualifying criteria to use them are.
>
> (Any wisdom for me here, Markus?)

From Kevin's cover letter:

    Some QMP command handlers can block the main loop for a relatively
    long time, for example because they perform some I/O.  This is quite
    nasty.  Allowing such handlers to run in a coroutine where they can
    yield (and therefore release the BQL) while waiting for an event
    such as I/O completion solves the problem.

Running in a coroutine is not a replacement for jobs.  Monitor commands
continue to run one after the other, even with multiple monitors.  All
this does is letting monitor commands yield.

Running in a coroutine is opt-in, because we're scared of command code
misbehaving in coroutine context[*].  To opt-in, add

    'coroutine': true

to the command's QAPI schema.

Misbehaving command code should be rare.  The trouble is finding it.  If
we had a better handle on that, we could make running in a coroutine
opt-out.  Watch out for nested event loops.  Test thoroughly.

Questions?

[...]

[*] Discussed at some length in patch review:

    Message-ID: <874kwnvgad.fsf@dusky.pond.sub.org>
    https://lists.nongnu.org/archive/html/qemu-devel/2020-01/msg05015.html