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Re: [PATCH RFC C0/2] support allocation-map for block-dirty-bitmap-merge
From: |
Markus Armbruster |
Subject: |
Re: [PATCH RFC C0/2] support allocation-map for block-dirty-bitmap-merge |
Date: |
Wed, 28 Apr 2021 07:23:32 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
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