Daniel P. Berrangé <berra...@redhat.com> writes:
> On Tue, Oct 22, 2024 at 10:41:29AM +0200, Markus Armbruster wrote:
>> Peter Xu <pet...@redhat.com> writes:
>>
>> > Per previous discussion [1,2], this patch deprecates query-migrationthreads
>> > command.
>> >
>> > To summarize, the major reason of the deprecation is due to no sensible way
>> > to consume the API properly:
>> >
>> > (1) The reported list of threads are incomplete (ignoring destination
>> > threads and non-multifd threads).
>> >
>> > (2) For CPU pinning, there's no way to properly pin the threads with
>> > the API if the threads will start running right away after migration
>> > threads can be queried, so the threads will always run on the default
>> > cores for a short window.
>> >
>> > (3) For VM debugging, one can use "-name $VM,debug-threads=on" instead,
>> > which will provide proper names for all migration threads.
>> >
>> > [1] https://lore.kernel.org/r/20240930195837.825728-1-pet...@redhat.com
>> > [2] https://lore.kernel.org/r/20241011153417.516715-1-pet...@redhat.com
>> >
>> > Signed-off-by: Peter Xu <pet...@redhat.com>
[...]
>> > diff --git a/migration/threadinfo.c b/migration/threadinfo.c
>> > index 262990dd75..2867413420 100644
>> > --- a/migration/threadinfo.c
>> > +++ b/migration/threadinfo.c
>> > @@ -13,6 +13,7 @@
>> > #include "qemu/osdep.h"
>> > #include "qemu/queue.h"
>> > #include "qemu/lockable.h"
>> > +#include "qemu/error-report.h"
>> > #include "threadinfo.h"
>> >
>> > QemuMutex migration_threads_lock;
>> > @@ -52,6 +53,9 @@ MigrationThreadInfoList
>> > *qmp_query_migrationthreads(Error **errp)
>> > MigrationThread *thread = NULL;
>> >
>> > QEMU_LOCK_GUARD(&migration_threads_lock);
>> > +
>> > + warn_report("Command 'query-migrationthreads' is deprecated");
>>
>> We don't normally do this for QMP commands.
>>
>> Management applications can use -compat deprecated-input=reject to check
>> they're not sending deprecated commands or arguments.
>>
>> Suggest to drop.
>
> They could, but in practice I don't believe anything is doing this, so
> the warning message is a practical way to alert people to the usage.
Again, we not normally do this. What makes this one different?
Stepping onto my soapbox: if stuff going away surprisingly would cause
you enough inconvenience to make early warning desirable, testing with
suitable -compat is a lot more reliable than relying on warnings.
*Especially* when your automated testing files warnings unexamined
together with any other crap that may go to stderr, so your best chance
to notice the warning is in ad hoc manual testing of QEMU. Nobody does
that until after things broke.
[...]
