Subject: Re: [PATCH net-next] net/core: Enable socket busy polling on -RT

From: Kurt Kanzenbach <[email-masked]>
Date: Sat, 28 Oct 2023 12:09:18 +0200

Hi Florian,

On Fri Oct 27 2023, Florian Bezdeka wrote:
> On Tue, 2023-05-23 at 13:15 +0200, Kurt Kanzenbach wrote:
>> Busy polling is currently not allowed on PREEMPT_RT, because it disables
>> preemption while invoking the NAPI callback. It is not possible to acquire
>> sleeping locks with disabled preemption. For details see commit
>> 20ab39d13e2e ("net/core: disable NET_RX_BUSY_POLL on PREEMPT_RT").
>
> Is that something that we could consider as Bug-Fix for 6.1 and request
> a backport, or would you consider that as new feature?

IMO it is in category "never worked". Hence it is not stable material.

>
>> 
>> However, strict cyclic and/or low latency network applications may prefer busy
>> polling e.g., using AF_XDP instead of interrupt driven communication.
>> 
>> The preempt_disable() is used in order to prevent the poll_owner and NAPI owner
>> to be preempted while owning the resource to ensure progress. Netpoll performs
>> busy polling in order to acquire the lock. NAPI is locked by setting the
>> NAPIF_STATE_SCHED flag. There is no busy polling if the flag is set and the
>> "owner" is preempted. Worst case is that the task owning NAPI gets preempted and
>> NAPI processing stalls.  This is can be prevented by properly prioritising the
>> tasks within the system.
>> 
>> Allow RX_BUSY_POLL on PREEMPT_RT if NETPOLL is disabled. Don't disable
>> preemption on PREEMPT_RT within the busy poll loop.
>> 
>> Tested on x86 hardware with v6.1-RT and v6.3-RT on Intel i225 (igc) with
>> AF_XDP/ZC sockets configured to run in busy polling mode.
>
> That is exactly our use case as well and we would like to have it in
> 6.1. Any (technical) reasons that prevent a backport?

There is no technical reason which prevents a backport to v6.1. In fact,
we're using this with v6.1-RT LTS.

Thanks,
Kurt