linux-kernel - Re: [PATCH net-next 10/10] net/mlx5e: Use the 'num

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Message-ID: <20250910162310.GF882933@ziepe.ca>
Date: Wed, 10 Sep 2025 13:23:10 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: Tariq Toukan <tariqt@...dia.com>, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jiri@...nulli.us>, Jonathan Corbet <corbet@....net>,
	Leon Romanovsky <leon@...nel.org>,
	Saeed Mahameed <saeedm@...dia.com>, Mark Bloch <mbloch@...dia.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rdma@...r.kernel.org, bpf@...r.kernel.org,
	Gal Pressman <gal@...dia.com>, Cosmin Ratiu <cratiu@...dia.com>,
	Dragos Tatulea <dtatulea@...dia.com>, Jiri Pirko <jiri@...dia.com>
Subject: Re: [PATCH net-next 10/10] net/mlx5e: Use the 'num_doorbells'
 devlink param

On Wed, Sep 10, 2025 at 09:16:40AM -0700, Stanislav Fomichev wrote:

> > +   * - ``num_doorbells``
> > +     - driverinit
> > +     - This controls the number of channel doorbells used by the netdev. In all
> > +       cases, an additional doorbell is allocated and used for non-channel
> > +       communication (e.g. for PTP, HWS, etc.). Supported values are:
> > +       - 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`?

I expect it to be common that most deployment should continue to use
the historical value of num_doorbells = 0.

Certain systems with troubled CPUs will need to increase this, I don't
know if we yet fully understand what values these CPUs will need.

Nor do I think I'm permitted to say what CPUs are troubled :\

> 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?

It has a cost that should be minimized to not harm the current
majority of users.

Jason