netdev - Re: [PATCH net-next v3 3/4] Extend napi threaded polling to allow kthread based busy polling

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Message-ID: <92cdbb04-4a59-41c7-954a-676d21ebd8f5@intel.com>
Date: Wed, 5 Feb 2025 19:40:52 -0800
From: "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To: Samiullah Khawaja <skhawaja@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
	"David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>, <almasrymina@...gle.com>
CC: <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v3 3/4] Extend napi threaded polling to allow
 kthread based busy polling



On 2/4/2025 4:10 PM, 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.

Looks like this patch is changing the sysfs 'threaded' field from
boolean to an integer and value 2 is used to indicate threaded mode with
busypoll.
So I think the above comment should reflect that instead of just saying
enabled for both 'threaded' and 'threaded with busypoll'.

> 
> - 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.

When does STATE_THREADED_BUSY_POLL get unset? Can this only be unset 
from the userspace? Don't we need a timeout value to come out of 
busypoll mode if there is no traffic?