Re: [PATCH] migration: Remove interface query-migrationthreads

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> This reverts two commits:
>
> 671326201dac8fe91222ba0045709f04a8ec3af4
> 1b1f4ab69c41279a45ccd0d3178e83471e6e4ec1
>
> Meanwhile it adds an entry to removed-features.rst for the
> query-migrationthreads QMP command.
>
> This patch originates from another patchset [1] that wanted to cleanup the
> interface and add corresponding HMP command, as lots of things are missing
> in the query report; so far it only reports the main thread and multifd
> sender threads; all the rest migration threads are not reported, including
> multifd recv threads.
>
> As pointed out by Dan in the follow up discussions [1], the API is designed
> in an awkward way where CPU pinning may not cover the whole lifecycle of
> even the thread being reported.  When asked, we also didn't get chance to
> hear from the developer who introduced this feature to explain how this API
> can be properly used.
>
> OTOH, this feature from debugging POV isn't very helpful either, as all
> these information can be easily obtained by GDB.  Esepcially, if with
> "-name $VM,debug-threads=on" we do already have names for each migration
> threads (which covers more than multifd sender threads).
>
> So it looks like the API isn't helpful in any form as of now, besides it
> only adds maintenance burden to migration code, even if not much.
>
> Considering that so far there's totally no justification on how to use this
> interface correctly, let's remove this interface instead of cleaning it up.
>
> In this special case, we even go beyond normal deprecation procedure,
> because a deprecation process would only make sense when there are existing
> users. In this specific case, we expect zero serious users with this API.
>
> [1] https://lore.kernel.org/qemu-devel/20240930195837.825728-1-peterx@redhat.com/
>
> Cc: Jiang Jiacheng <jiangjiacheng@huawei.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Fabiano Rosas <farosas@suse.de>