Re: Question about QMP and BQL

[Markus Armbruster]

Kevin, any advice?

Fabiano Rosas <farosas@suse.de> writes:

> Is there a way to execute a long-standing QMP command outside of the
> BQL?
>
> The situation we're seeing is a slow query-block due to a slow system
> call (fstat over NFS) causing the main thread to spend too long
> holding the global mutex and locking up the vcpu thread when it goes
> out of the guest for MMIO.
>
> The call chain for QMP is:
>
> qmp_query_block
> bdrv_query_info
> bdrv_block_device_info
> bdrv_query_image_info
> bdrv_do_query_node_info
> bdrv_get_allocated_file_size
> bdrv_poll_co <- Waiting with qemu_global_mutex locked
>
> [coroutine] bdrv_co_get_allocated_file_size_entry
> bdrv_co_get_allocated_file_size
> raw_co_get_allocated_file_size
> fstat <- SLOW!
>
> The closest I got was moving the coroutine into a separate iothread,
> unlocking the global mutex and releasing the bdrv aio_context around
> aio_poll. It feels wrong though because we're technically still
> operating on the block state but not holding the context.
>
> Is there a more standard way if doing this? Is it possible at all?