Re: [PATCH net-next 10/10] net/mlx5e: Use the 'num_doorbells' devlink param

[Cosmin Ratiu <cratiu@nvidia.com>]

On Wed, 2025-09-10 at 09:16 -0700, Stanislav Fomichev wrote:
> > +       - 0: No channel-specific doorbells, use the global one for
> > everything.
> > +       - [1, max_num_channels]: Spread netdev channels equally
> > across these
> > +         doorbells.
> 
> Do you have any guidance on this number? Why would the user want
> `num_doorbells < num_doorbells` vs `num_doorbells == num_channels`?
> 
> IOW, why not allocate the same number of doorbells as the number of
> channels and do it unconditionally without devlink param? Are extra
> doorbells causing any overhead in the non-contended case?

In most cases, additional doorbells are an overhead and not required.
For the last 10+ years, mlx5 has been running with a single doorbell
for all channels. But as the number of cores and channels grew,
bottlenecks were discovered on some platforms. Thus the need for this
series.

This series proposes 8 as the new default and we expect nobody would
need to touch this knob except in extreme cases, but I think it's nice
for it to exist. Regression testing showed little to no impact on most
platforms where 1 doorbell was enough, and a significant improvement on
platforms that showed MMIO bottlenecks.

I'll add some numbers in the next version's cover letter.

Cosmin.