netdev - Re: [PATCH RFC]: napi

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Date:	Thu, 26 Jul 2007 14:38:09 -0700
From:	"Michael Chan" <mchan@...adcom.com>
To:	"David Miller" <davem@...emloft.net>
cc:	"netdev" <netdev@...r.kernel.org>, shemminger@...ux-foundation.org,
	jgarzik@...ox.com, hadi@...erus.ca, rusty@...tcorp.com.au
Subject: Re: [PATCH RFC]: napi_struct V4

On Thu, 2007-07-26 at 00:15 -0700, David Miller wrote:

> The netpoll code has to take that anyways in order to call
> into ->hard_start_xmit() to send out the packet it has
> pending, I'm leveraging that as a synchronization mechanism
> in the drivers because the locking options are limited
> given that netpoll can try to do this in any context whatsoever.
> 
> > There is a measurable difference in oprofile.  When passing small
> > packets, there's a non-trivial difference in throughput.
> 
> Then please help come up with an alternate scheme, because these
> NAPI changes fix real limitations and bugs in the current code
> and unless we fix netpoll too we can't move forward.
> 

I'm not very familiar with netpoll, but I am guessing that we need to
prevent concurrent calls to the TX cleanup code.  For maximum
flexibility, we should let ->poll_controller() handle the locking as the
driver knows what kind of locking is best.

If the driver wants a simple solution, it can do what you did in the
patch:  wrap the tx cleanup code with netif_tx_lock() and
netif_tx_unlock().

If a NAPI driver wants to be more clever, it can do something such as
this in tg3's poll_controller:

if (netif_rx_schedule_prep(dev, &tp->napi)) {
	tg3_tx(tp);
	netif_poll_enable(tp->napi);
}

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