netdev - Re: [RFC PATCH net-next 1/2] net: Use SMP threads for backlog NAPI.

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Date: Wed, 23 Aug 2023 15:35:41 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>,  Jakub Kicinski <kuba@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Wander
 Lairson Costa <wander@...hat.com>
Subject: Re: [RFC PATCH net-next 1/2] net: Use SMP threads for backlog NAPI.

On Mon, 2023-08-14 at 11:35 +0200, Sebastian Andrzej Siewior wrote:
> @@ -4781,7 +4733,7 @@ static int enqueue_to_backlog(struct sk_buff *skb, int cpu,
>  		 * We can use non atomic operation since we own the queue lock
>  		 */
>  		if (!__test_and_set_bit(NAPI_STATE_SCHED, &sd->backlog.state))
> -			napi_schedule_rps(sd);
> +			__napi_schedule_irqoff(&sd->backlog);
>  		goto enqueue;
>  	}
>  	reason = SKB_DROP_REASON_CPU_BACKLOG;

I *think* that the above could be quite dangerous when cpu ==
smp_processor_id() - that is, with plain veth usage.

Currently, each packet runs into the rx path just after
enqueue_to_backlog()/tx completes.

With this patch there will be a burst effect, where the backlog thread
will run after a few (several) packets will be enqueued, when the
process scheduler will decide - note that the current CPU is already
hosting a running process, the tx thread.

The above can cause packet drops (due to limited buffering) or very
high latency (due to long burst), even in non overload situation, quite
hard to debug.

I think the above needs to be an opt-in, but I guess that even RT
deployments doing some packet forwarding will not be happy with this
on.

Cheers,

Paolo