[PATCH rfc] nvme: support io stats on the mpath device

[Keith Busch]

On Thu, Sep 29, 2022 at 12:59:46PM +0300, Sagi Grimberg wrote:
> > 3. Do you have some performance numbers (we're touching the fast path
> > here) ?
> 
> This is pretty light-weight, accounting is per-cpu and only wrapped by
> preemption disable. This is a very small price to pay for what we gain.

It does add up, though, and some environments disable stats to skip the
overhead. At a minimum, you need to add a check for blk_queue_io_stat() before
assuming you need to account for stats.

Instead of duplicating the accounting, could you just have the stats file report
the sum of its hidden devices?