Re: [PATCH 0/2] abstract napi tracking strategy

[Olivier Langlois <olivier@trillion01.com>]

On Tue, 2024-08-13 at 12:33 -0600, Jens Axboe wrote:
> On 8/13/24 10:44 AM, Olivier Langlois wrote:
> > the actual napi tracking strategy is inducing a non-negligeable
> > overhead.
> > Everytime a multishot poll is triggered or any poll armed, if the
> > napi is
> > enabled on the ring a lookup is performed to either add a new napi
> > id into
> > the napi_list or its timeout value is updated.
> > 
> > For many scenarios, this is overkill as the napi id list will be
> > pretty
> > much static most of the time. To address this common scenario, a
> > new
> > abstraction has been created following the common Linux kernel
> > idiom of
> > creating an abstract interface with a struct filled with function
> > pointers.
> > 
> > Creating an alternate napi tracking strategy is therefore made in 2
> > phases.
> > 
> > 1. Introduce the io_napi_tracking_ops interface
> > 2. Implement a static napi tracking by defining a new
> > io_napi_tracking_ops
> 
> I don't think we should create ops for this, unless there's a strict
> need to do so. Indirect function calls aren't cheap, and the CPU side
> mitigations for security issues made them worse.
> 
> You're not wrong that ops is not an uncommon idiom in the kernel, but
> it's a lot less prevalent as a solution than it used to. Exactly
> because
> of the above reasons.
> 
if indirection is a very big deal, it might be possible to remove one
level of indirection.

I did entertain the idea of making copies of the io_napi_tracking_ops
structs instead of storing their addresses. I did not kept this option
because of the way that I did implement io_napi_get_tracking()...

but if this would be an acceptable compromise, this is definitely
something possible.