netdev - Re: [RFC] loopback: optimization

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Message-ID: <491228C8.3010100@cosmosbay.com>
Date:	Thu, 06 Nov 2008 00:14:16 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] loopback: optimization

Stephen Hemminger a écrit :
> Convert loopback device from using common network queues to a per-cpu
> receive queue with NAPI. This gives a small 1% performance gain when
> measured over 5 runs of tbench. Not sure if it's worth bothering
> though.
> 
> Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> 
> 
> --- a/drivers/net/loopback.c	2008-11-04 15:36:29.000000000 -0800
> +++ b/drivers/net/loopback.c	2008-11-05 10:00:20.000000000 -0800
> @@ -59,7 +59,10 @@
>  
> +/* Special case version of napi_schedule since loopback device has no hard irq */
> +void napi_schedule_irq(struct napi_struct *n)
> +{
> +	if (napi_schedule_prep(n)) {
> +		list_add_tail(&n->poll_list, &__get_cpu_var(softnet_data).poll_list);
> +		__raise_softirq_irqoff(NET_RX_SOFTIRQ);
> +	}
> +}
> +

Stephen, I dont get it.

Sure loopback device cannot generate hard irqs, but what prevent's a real hardware
interrupt to call NIC driver that can call napi_schedule() and corrupt softnet_data.poll_list ?

Why not using a queue dedicated on loopback directly in cpu_var(softnet_data) ?

(ie not using a napi structure for each cpu and each loopback dev)

This queue would be irq safe yes.

net_rx_action could handle this list without local_irq_disable()/local_irq_enable() games.

Hum, maybe complex for loopback_dev_stop() to purge all queues without interfering with other namespaces.




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