Re: [PATCH v4 1/1] nvme: multipath: Implemented new iopolicy "queue-depth"

[Sagi Grimberg]

On 22/05/2024 19:29, Keith Busch wrote:
> On Wed, May 22, 2024 at 12:23:51PM -0400, John Meneghini wrote:
> > On 5/22/24 11:56, Keith Busch wrote:
> > > On Wed, May 22, 2024 at 11:42:12AM -0400, John Meneghini wrote:
> > > > +static void nvme_subsys_iopolicy_update(struct nvme_subsystem *subsys, int iopolicy)
> > > > +{
> > > > +	struct nvme_ctrl *ctrl;
> > > > +	int old_iopolicy = READ_ONCE(subsys->iopolicy);
> > > > +
> > > > +	WRITE_ONCE(subsys->iopolicy, iopolicy);
> > > > +
> > > > +	/* iopolicy changes reset the counters and clear the mpath by design */
> > > > +	mutex_lock(&nvme_subsystems_lock);
> > > > +	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
> > > > +		atomic_set(&ctrl->nr_active, 0);
> > > Can you me understand why this is a desirable feature? Unless you
> > > quiesce everything at some point, you'll always have more unaccounted
> > > requests on whichever path has higher latency. That sounds like it
> > > defeats the goals of this io policy.
> > This is true. And as a matter of practice I never change the IO policy when IOs are in flight.  I always stop the IO first.
> > But we can't stop any user from changing the IO policy again and again.  So I'm not sure what to do.
> >
> > If you'd like I add the 'if (old_iopolicy == iopolicy) return;' here, but
> > that's not going to solve the problem of inaccurate counters when users
> > start flipping io policies around. with IO inflight. There is no
> > synchronization between io submission across controllers and changing the
> > policy so I expect changing between round-robin and queue-depth with IO
> > inflight suffers from the same problem... though not as badly.
> >
> > I'd rather take this patch now and figure out how to fix the problem with
> > another patch in the future.  Maybe we can check the io stats and refuse to
> > change the policy of they are not zero....
> The idea of tagging the nvme_req()->flags on submission means the
> completion handling the nr_active counter is symmetric with the
> submission side: you don't ever need to reset nr_active because
> everything is accounted for.

Agree. We should probably remove it from the patch.