[RFC 1/3] nvme: skip block cgroups for passthrough commands

[Keith Busch]

On Wed, May 03, 2023 at 07:04:14AM +0200, Christoph Hellwig wrote:
> On Mon, May 01, 2023 at 08:33:04AM -0700, Keith Busch wrote:
> > From: Keith Busch <kbusch at kernel.org>
> > 
> > Passthrough requests don't go through the submit_bio() path, so all the
> > overhead of setting up the bio's cgroup is wasted cycles. Provide a path
> > to skip this setup.
> 
> These days we should not need to set bi_bdev at all for passthrough,
> so I think we can just drop the assingment.

We can't really skip it for polling since that needs a bio with a
bdev. I'll take another shot at detandling that requirement.
 
> But instead of just optimizing for passthrough we really need to optimize
> this assignment and get rid of the cost entirely.  What is so expensive
> about the cgroup lookup?  Is this even for a device that uses cgroups
> at all, or could we come up with a flag to bypass all the lookup unless
> cgroup are anbled?

It's not super expensive. It's just unnecessary for this usage and
this seemed like the easiest way to avoid it.