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

[Keith Busch]

On Thu, Sep 29, 2022 at 07:32:39PM +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.
> 
> But QUEUE_FLAG_IO_STAT is set by nvme-mpath itself?
> You mean disable IO stats in runtime?

Yes, the user can disable it at any time. That actually makes things a bit
tricky since it can be enabled at the start of an IO and disabled by the time
it completes.