linux-kernel - Re: [RFC] HOWTO use NAPI to reduce TX interrupts

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Date:	Sat, 19 Aug 2006 13:25:18 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Stephen Hemminger <shemminger@...l.org>
Cc:	linuxppc-dev@...abs.org, akpm@...l.org,
	James K Lewis <jklewis@...ibm.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Jeff Garzik <jgarzik@...ox.com>,
	Jens Osterkamp <Jens.Osterkamp@...ibm.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [RFC] HOWTO use NAPI to reduce TX interrupts

On Sunday 20 August 2006 03:31, Stephen Hemminger wrote:
> 
> The reason reclaim via poll() is efficient is because it avoid causing a 
> softirq that is
> necessary when skb_free_irq() is done. Instead it reuses the softirq 
> from the poll() routine. 

Ok, I completely missed this point so far, thanks for the info.

> Like all Rx NAPI, using poll() for reclaim means: 
>     + aggregating multiple frames in one irq
>     - increased overhead of twiddling with the IRQ mask
>     - more ways to get driver stuck

What is the best way to treat the IRQ mask for TX interrupts?
I guess it should be roughly:

- off when we expect ->poll() to be called, i.e. after calling
  netif_rx_schedule() or returning after a partial rx from poll().
- off when there are no packets left in the TX queue
- on while RX interrupts are on and we're waiting for packets
  to be transmitted.

> Some drivers do all their irq work in the poll() routine (including PHY 
> handling).
> This is good if reading the IRQ status does an auto mask operation.
> 
> The whole NAPI documentation area is a mess and needs a good writer
> to do some major restructuring. It should also be split into reference 
> information,
> tutorial and guide sections.

I won't be able to do that work, I'm neither a good writer nor a networking
person.

Do you think we should still merge a section like the text I wrote up, even
if it makes the text even less well structured? Should I maybe add it
somewhere else than the appendix?

	Arnd <><
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