netdev - Re: [PATCH net-next 0/3] Add support to do threaded napi busy poll

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Message-ID: <20250102215642.21da3755@dsl-u17-10>
Date: Thu, 2 Jan 2025 21:56:42 +0000
From: David Laight <david.laight.linux@...il.com>
To: Samiullah Khawaja <skhawaja@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, "David S . Miller "
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 0/3] Add support to do threaded napi busy poll

On Thu,  2 Jan 2025 19:12:24 +0000
Samiullah Khawaja <skhawaja@...gle.com> wrote:

> Extend the already existing support of threaded napi poll to do continuous
> busypolling.
> 
> This is used for doing continuous polling of napi to fetch descriptors from
> backing RX/TX queues for low latency applications. Allow enabling of threaded
> busypoll using netlink so this can be enabled on a set of dedicated napis for
> low latency applications.
> 
> Currently threaded napi is only enabled at device level using sysfs. Add
> support to enable/disable threaded mode for a napi individually. This can be
> done using the netlink interface. Add `set_threaded` op in netlink spec that
> allows setting the `threaded` attribute of a napi.
> 
> Extend the threaded attribute in napi struct to add an option to enable
> continuous busy polling. Extend the netlink and sysfs interface to allow
> enabled/disabling threaded busypolling at device or individual napi level.
> 
> Once threaded busypoll on a napi is enabled, depending on the application
> requirements the polling thread can be moved to dedicated cores. We used this
> for AF_XDP usecases to fetch packets from RX queues to reduce latency.

I think it would be more generally useful to be able to set the priority
of the NAPI thread.
'busypolling' is just an extreme version of a high priority thread.

We've had to use threaded NAPI and a low SCHED_FIFO priority in order to
clear the RX queues without the softint code locking out a other SCHED_FIFO
user threads for 20ms+.
That is currently rather a pain to setup.

	David