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

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Date: Thu, 25 May 2023 13:16:46 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Kurt Kanzenbach <kurt@...utronix.de>, "David S. Miller"
	 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	 <kuba@...nel.org>
Cc: netdev@...r.kernel.org, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>,  juri.lelli@...hat.com
Subject: Re: [PATCH net-next] net/core: Enable socket busy polling on -RT

Hi,

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").
> 
> 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.
> 
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>

The patch looks reasonable to me, but it would be great to hear a
second opinion from someone from RT side.

CC: Juri


Thanks!

Paolo