Subject: Re: [PATCH net-next 3/3] Extend napi threaded polling to allow kthread based busy polling

From: Stanislav Fomichev <[email-masked]>
Date: Thu, 2 Jan 2025 13:28:57 -0800

On 01/02, Samiullah Khawaja wrote:
> Add a new state to napi state enum:
> 
> - 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 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 we are
>   supposed to busy poll for each napi.
> 
> - When napi is scheduled with STATE_SCHED_THREADED and associated
>   kthread is woken up, the kthread owns the context. If
>   NAPI_STATE_THREADED_BUSY_POLL and NAPI_SCHED_THREADED both are set
>   then it means that we can busy poll.
> 
> - To keep busy polling and to avoid scheduling of the interrupts, the
>   napi_complete_done returns false when both SCHED_THREADED and
>   THREADED_BUSY_POLL flags are set. Also napi_complete_done returns
>   early to avoid the STATE_SCHED_THREADED being unset.
> 
> - If at any point STATE_THREADED_BUSY_POLL is unset, the
>   napi_complete_done will run and unset the SCHED_THREADED bit also.
>   This will make the associated kthread go to sleep as per existing
>   logic.
> 
> Signed-off-by: Samiullah Khawaja <[email-masked]>
> Reviewed-by: Willem de Bruijn <[email-masked]>
> ---
>  Documentation/ABI/testing/sysfs-class-net     |  3 +-
>  Documentation/netlink/specs/netdev.yaml       |  5 +-
>  .../net/ethernet/atheros/atl1c/atl1c_main.c   |  2 +-
>  include/linux/netdevice.h                     | 24 +++++--
>  net/core/dev.c                                | 72 ++++++++++++++++---
>  net/core/net-sysfs.c                          |  2 +-
>  net/core/netdev-genl-gen.c                    |  2 +-
>  7 files changed, 89 insertions(+), 21 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:	[email-masked]
>  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.
>  		== ==================================
> diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
> index aac343af7246..9c905243a1cc 100644
> --- a/Documentation/netlink/specs/netdev.yaml
> +++ b/Documentation/netlink/specs/netdev.yaml
> @@ -272,10 +272,11 @@ attribute-sets:
>          name: threaded
>          doc: Whether the napi is configured to operate in threaded polling
>               mode. If this is set to `1` then the NAPI context operates
> -             in threaded polling mode.
> +             in threaded polling mode. If this is set to `2` then the NAPI
> +             kthread also does busypolling.
>          type: u32
>          checks:
> -          max: 1
> +          max: 2
>    -

I'd vote for a separate threaded-busy-poll parameter (and separate doc)
instead of overloading 'threaded' bool. But if you prefer to
have a single argument, let's at least change it to enum with proper
values for busy and non-busy modes instead of magic numbers?