Re: [Qemu-devel] [PATCH 07/19] block: Really pause block jobs on drain

[Paolo Bonzini]

On 12/04/2018 16:25, Kevin Wolf wrote:
> This is already the order we have there. What is probably different from
> what you envision is that after the parents have concluded, we still
> check that they are still quiescent in every iteration.

Yes, and that's the quadratic part.

> What we could do easily is introducing a bool bs->quiesce_concluded or
> something that bdrv_drain_poll() sets to true the first time that it
> returns false. It also gets a shortcut so that it returns false
> immediately if bs->quiesce_concluded is true. The field is reset in
> bdrv_do_drained_end() when bs->quiesce_counter reaches 0.

Or bs->quiescent, for the sake of bikeshedding.

> That would be a rather simple optimisation that could be done as the
> final patch in this series, and would ensure that we don't recurse back
> to parents that are already quiescent.
> 
> Would you be happy with this change?

Yes, I'll leave the organization of the series to you of course.

Paolo