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

[Florian Bezdeka <florian.bezdeka@siemens.com>]

Hi Kurt,

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?

> 
> 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?

As some time has already passed since patch submission I will not cut
the rest...

Best regards,
Florian

> 
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
> ---
> 
> Changes since RFC:
> 
>  * Commit message
> 
> Previous version:
> 
>  * https://lore.kernel.org/all/20230517110950.78322-1-kurt@linutronix.de/
> 
>  net/Kconfig    | 2 +-
>  net/core/dev.c | 9 ++++++---
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/net/Kconfig b/net/Kconfig
> index 7d39c1773eb4..2fb25b534df5 100644
> --- a/net/Kconfig
> +++ b/net/Kconfig
> @@ -324,7 +324,7 @@ config CGROUP_NET_CLASSID
>  
>  config NET_RX_BUSY_POLL
>  	bool
> -	default y if !PREEMPT_RT
> +	default y if !PREEMPT_RT || (PREEMPT_RT && !NETCONSOLE)
>  
>  config BQL
>  	bool
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b3c13e041935..3393c2f3dbe8 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6197,7 +6197,8 @@ void napi_busy_loop(unsigned int napi_id,
>  	if (!napi)
>  		goto out;
>  
> -	preempt_disable();
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_disable();
>  	for (;;) {
>  		int work = 0;
>  
> @@ -6239,7 +6240,8 @@ void napi_busy_loop(unsigned int napi_id,
>  		if (unlikely(need_resched())) {
>  			if (napi_poll)
>  				busy_poll_stop(napi, have_poll_lock, prefer_busy_poll, budget);
> -			preempt_enable();
> +			if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +				preempt_enable();
>  			rcu_read_unlock();
>  			cond_resched();
>  			if (loop_end(loop_end_arg, start_time))
> @@ -6250,7 +6252,8 @@ void napi_busy_loop(unsigned int napi_id,
>  	}
>  	if (napi_poll)
>  		busy_poll_stop(napi, have_poll_lock, prefer_busy_poll, budget);
> -	preempt_enable();
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_enable();
>  out:
>  	rcu_read_unlock();
>  }
> -- 
> 2.30.2
>