[PATCH 2/3] nvme: multipath: only update ctrl->nr_active when using queue-depth iopolicy

[Christoph Hellwig]

On Tue, Nov 07, 2023 at 02:53:48PM -0700, Keith Busch wrote:
> On Tue, Nov 07, 2023 at 04:23:30PM -0500, Ewan D. Milne wrote:
> > The atomic updates of ctrl->nr_active are unnecessary when using
> > numa or round-robin iopolicy, so avoid that cost on a per-request basis.
> > Clear nr_active when changing iopolicy and do not decrement below zero.
> > (This handles changing the iopolicy while requests are in flight.)
> 
> Oh, here's restricting it to that policy. Any reason not to fold it in
> the first one?

It should, and I agree with all the other comments.

But I'm also pretty deeply unhappy with the whole thing.  This is a
controller-wise atomic taken for every I/O.  How slow are the subsystems
people want to use it for?  And is a global max active really the
right measure, or would e a per-cpu, or at least batched per-cpu as
used by the percpu counters by a better option?