Subject: Re: [PATCH net-next 3/4] net: add skb_defer_max sysctl

From: Jakub Kicinski <[email-masked]>
Date: Mon, 16 May 2022 13:39:41 -0700

On Sun, 15 May 2022 21:24:55 -0700 Eric Dumazet wrote:
> @@ -6494,16 +6495,21 @@ void skb_attempt_defer_free(struct sk_buff *skb)
>  	int cpu = skb->alloc_cpu;
>  	struct softnet_data *sd;
>  	unsigned long flags;
> +	unsigned int defer_max;
>  	bool kick;
>  
>  	if (WARN_ON_ONCE(cpu >= nr_cpu_ids) ||
>  	    !cpu_online(cpu) ||
>  	    cpu == raw_smp_processor_id()) {
> -		__kfree_skb(skb);
> +nodefer:	__kfree_skb(skb);
>  		return;
>  	}
>  
>  	sd = &per_cpu(softnet_data, cpu);
> +	defer_max = READ_ONCE(sysctl_skb_defer_max);
> +	if (READ_ONCE(sd->defer_count) >= defer_max)
> +		goto nodefer;
> +
>  	/* We do not send an IPI or any signal.
>  	 * Remote cpu will eventually call skb_defer_free_flush()
>  	 */
> @@ -6513,11 +6519,8 @@ void skb_attempt_defer_free(struct sk_buff *skb)
>  	WRITE_ONCE(sd->defer_list, skb);
>  	sd->defer_count++;
>  
> -	/* kick every time queue length reaches 128.
> -	 * This condition should hardly be hit under normal conditions,
> -	 * unless cpu suddenly stopped to receive NIC interrupts.
> -	 */
> -	kick = sd->defer_count == 128;
> +	/* Send an IPI every time queue reaches half capacity. */
> +	kick = sd->defer_count == (defer_max >> 1);

nit: it will behave a little strangely for defer_max == 1
we'll let one skb get onto the list and free the subsequent 
skbs directly but we'll never kick the IPI

Moving the sd->defer_count++; should fix it and have no significant
side effects. I think.