netdev - Re: [RFC] loopback: optimization

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Date:	Wed, 5 Nov 2008 16:42:51 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] loopback: optimization

On Thu, 06 Nov 2008 00:14:16 +0100
Eric Dumazet <dada1@...mosbay.com> wrote:

> 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.

I did try a workqueue and kthread version previously, but they both had much worse
performance. Forgot that the NAPI schedule is shared, so yes that would have to locked.

Doing it purely for loopback would mean using a tasklet or another softirq.
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