netdev - Re: [PATCH net-next v5] Add support to set napi threaded for individual napi

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Message-ID: <aAxFmKo2cmLUmqAJ@LQ3V64L9R2>
Date: Fri, 25 Apr 2025 19:31:52 -0700
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Samiullah Khawaja <skhawaja@...gle.com>,
	"David S . Miller " <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	almasrymina@...gle.com, willemb@...gle.com, mkarsten@...terloo.ca,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next v5] Add support to set napi threaded for
 individual napi

On Fri, Apr 25, 2025 at 05:42:51PM -0700, Jakub Kicinski wrote:
> On Wed, 23 Apr 2025 20:14:13 +0000 Samiullah Khawaja wrote:
> > A net device has a threaded sysctl that can be used to enable threaded
> > napi polling on all of the NAPI contexts under that device. Allow
> > enabling threaded napi polling at individual napi level using netlink.
> > 
> > Extend the netlink operation `napi-set` and allow setting the threaded
> > attribute of a NAPI. This will enable the threaded polling on a napi
> > context.
> 
> I think I haven't replied to you on the config recommendation about
> how global vs per-object config should behave. I implemented the
> suggested scheme for rx-buf-len to make sure its not a crazy ask:
> https://lore.kernel.org/all/20250421222827.283737-1-kuba@kernel.org/
> and I do like it more.
> 
> Joe, Stanislav and Mina all read that series and are CCed here.
> What do y'all think? Should we make the threaded config work like
> the rx-buf-len, if user sets it on a NAPI it takes precedence
> over global config? Or stick to the simplistic thing of last
> write wins?

For the per-NAPI defer-hard-irqs (for example):
  - writing to the NIC-wide sysfs path overwrites all of the
    individual NAPI settings to be the global setting written
  - writing to an individual NAPI, though, the setting takes
    precedence over the global

So, if you wrote 100 to the global path, then 5 to a specific NAPI,
then 200 again to the global path, IIRC the NAPI would go through:
  - being set to 100 (from the global path write)
  - being set to 5 (for its NAPI specific write)
  - being set to 200 (from the final global path write)

The individual NAPI setting takes precedence over the global
setting; but the individual setting is re-written when the global
value is adjusted.

Can't tell if that's clear or if I just made it worse ;)

Anyway: I have a preference for consistency when possible, so IMHO,
it would be nice if:
  - Writing to NIC-wide threaded set all NAPIs to the value written
    to the NIC-wide setting
  - Individual NAPIs can have threaded enabled/disabled, which takes
    precedence over global

But IDK if that's realistic/desirable/or even what everyone else
prefers :)