netdev - Re: [PATCH RFC]: napi

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Date:	Wed, 25 Jul 2007 09:56:54 +0100
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, jgarzik@...ox.com, hadi@...erus.ca,
	rusty@...tcorp.com.au
Subject: Re: [PATCH RFC]: napi_struct V4

On Wed, 25 Jul 2007 01:31:54 -0700 (PDT)
David Miller <davem@...emloft.net> wrote:

> 
> We're getting there, slowly...
> 
> 1) netif_napi_init() is added, the workqueue/requeue stuff
>    as discussed is not needed so you won't see that here
> 
> 2) all cases where netif_rx_complete() are invoked in contexts
>    where cpu interrupts are known to be disabled are replaced
>    with __netif_rx_complete()
> 
> Most drivers are in good shape, although some still have very
> questionable netif_rx_complete() handling, in that racy area that
> Rusty and myself were discussing today.
> 
> My inclination is to wrap those sequences around with an IRQ
> safe spinlock to fix the race provably, and then if driver
> authors want to optimize that away with techniques like those
> that tg3, bnx2, sky2, skge et al. use, that's fine but can
> be done later.
> 
> To some extent it's mechanical work if you know what to look
> for, so any relative patches for that stuff would be much
> appreciated and I'll integrate such patches rapidly and without
> delay.
> 
> Besides that the only major issue is netpoll and I have some
> ideas on how to handle that, which I'll try to implement
> tonight and tomorrow.
> 
> Another thing that's really apparent now is all the wacky
> napi->weight values various drivers use.  Just grep for
> netif_napi_init() in the patch or a patched tree to see what
> I mean.  So much of it doesn't make any sense and I'm tempted
> to just remove the argument and make everyone use 32 or 64
> or something like that :-)  Or, default to some value across
> the board, and let drivers override that on a case by case
> basis with a BIG FAT COMMENT above the override describing
> why the different value is being used and precisely what
> tests were performed to validate that different value.
> 

The usage of NAPI on 8139cp and 8139too seems dodgy;
these drivers expect this to work:

		local_irq_save(flags);
		cpw16_f(IntrMask, cp_intr_mask);
		__netif_rx_complete(dev);
		local_irq_restore(flags);

It works on SMP only because if poll races with IRQ, 
the IRQ is not masked or cleared so the IRQ will get restarted.

Better would be to change it to:
		spin_lock_irqsave(&cp->lock, flags);
		cpw16_f(IntrMask, cp_intr_mask);
		__netif_rx_complete(dev);
		spin_unlock_irqrestore(&cp->lock, flags);

Which actually is same code on UP.
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