netdev - Re: [PATCH net-next v7 1/2] Extend napi threaded polling to allow kthread based busy polling

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Message-ID: <aKy9z1oephUbAr_E@mini-arch>
Date: Mon, 25 Aug 2025 12:47:27 -0700
From: Stanislav Fomichev <stfomichev@...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>,
	almasrymina@...gle.com, willemb@...gle.com, mkarsten@...terloo.ca,
	Joe Damato <joe@...a.to>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v7 1/2] Extend napi threaded polling to allow
 kthread based busy polling

On 08/24, Samiullah Khawaja wrote:
> Add a new state to napi state enum:
> 
> - NAPI_STATE_THREADED_BUSY_POLL
>   Threaded busy poll is enabled/running for this napi.
> 
> Following changes are introduced in the napi scheduling and state logic:
> 
> - When threaded busy poll is enabled through sysfs or netlink it also
>   enables NAPI_STATE_THREADED so a kthread is created per napi. It also
>   sets NAPI_STATE_THREADED_BUSY_POLL bit on each napi to indicate that
>   it is going to busy poll the napi.
> 
> - When napi is scheduled with NAPI_STATE_SCHED_THREADED and associated
>   kthread is woken up, the kthread owns the context. If
>   NAPI_STATE_THREADED_BUSY_POLL and NAPI_STATE_SCHED_THREADED both are
>   set then it means that kthread can busy poll.
> 
> - To keep busy polling and to avoid scheduling of the interrupts, the
>   napi_complete_done returns false when both NAPI_STATE_SCHED_THREADED
>   and NAPI_STATE_THREADED_BUSY_POLL flags are set. Also
>   napi_complete_done returns early to avoid the
>   NAPI_STATE_SCHED_THREADED being unset.
> 
> - If at any point NAPI_STATE_THREADED_BUSY_POLL is unset, the
>   napi_complete_done will run and unset the NAPI_STATE_SCHED_THREADED
>   bit also. This will make the associated kthread go to sleep as per
>   existing logic.
> 
> Signed-off-by: Samiullah Khawaja <skhawaja@...gle.com>
> Reviewed-by: Willem de Bruijn <willemb@...gle.com>
> 
> ---
>  Documentation/ABI/testing/sysfs-class-net |  3 +-
>  Documentation/netlink/specs/netdev.yaml   |  5 +-
>  Documentation/networking/napi.rst         | 63 +++++++++++++++++++++-
>  include/linux/netdevice.h                 | 11 +++-
>  include/uapi/linux/netdev.h               |  1 +
>  net/core/dev.c                            | 66 +++++++++++++++++++----
>  net/core/dev.h                            |  3 ++
>  net/core/net-sysfs.c                      |  2 +-
>  net/core/netdev-genl-gen.c                |  2 +-
>  tools/include/uapi/linux/netdev.h         |  1 +
>  10 files changed, 142 insertions(+), 15 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-net b/Documentation/ABI/testing/sysfs-class-net
> index ebf21beba846..15d7d36a8294 100644
> --- a/Documentation/ABI/testing/sysfs-class-net
> +++ b/Documentation/ABI/testing/sysfs-class-net
> @@ -343,7 +343,7 @@ Date:		Jan 2021
>  KernelVersion:	5.12
>  Contact:	netdev@...r.kernel.org
>  Description:
> -		Boolean value to control the threaded mode per device. User could
> +		Integer value to control the threaded mode per device. User could
>  		set this value to enable/disable threaded mode for all napi
>  		belonging to this device, without the need to do device up/down.
>  
> @@ -351,4 +351,5 @@ Description:
>  		== ==================================
>  		0  threaded mode disabled for this dev
>  		1  threaded mode enabled for this dev
> +		2  threaded mode enabled, and busy polling enabled.

I might have asked already but forgot the answer: any reason we keep
extending sysfs? With a proper ynl control over per-queue settings,
why do we want an option to enable busy-polling threaded mode for the
whole device?