netdev - Re: [PATCH 3/16] Spidernet RX Locking

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Date:	Sat, 09 Dec 2006 09:47:05 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linas Vepstas <linas@...tin.ibm.com>
Cc:	Jeff Garzik <jgarzik@...ox.com>, Andrew Morton <akpm@...l.org>,
	Arnd Bergmann <arnd@...db.de>, netdev@...r.kernel.org,
	James K Lewis <jklewis@...ibm.com>, linuxppc-dev@...abs.org
Subject: Re: [PATCH 3/16] Spidernet RX Locking

A spinlock is expensive in the fast path, which is why Jeff says it's
invasive.

> spider_net_decode_one_descr() is called from
> spider_net_poll() (which is the netdev->poll callback)
> and also from spider_net_handle_rxram_full(). 
> 
> The rxramfull routine is called from a tasklet that
> is fired off after a "RX ram full" interrupt is receved.
> This interrupt is generated when the hardware runs out
> of space to store incoming packets. We are seeing this
> interrupt fire when the CPU is heavily loaded, and a
> lot of traffic is being fired at the device.

How often does that interrupt happen in that case ?

A better approach is to keep the fast path (ie. poll()) lockless, and in
handle_rxram_full(), the slow path, protect against poll using
netif_disable_poll(). Though that means using a work queue, not a
tasklet, since it needs to schedule.

> > and what other 
> > non-sledgehammer approaches were discarded before arriving at this one?
> 
> Well, I'm not that good at kernel programming, so I guess
> I did not perceive this as a "sledgehammer."  And alternative
> approach is to simply ignore the rxramfull interrupt entirely,
> and depend on poll() do all the work.   I'll try this shortly.

or you can schedule rx work from the rxramfull interrupt after setting a
"something bad happened" flag. Then, poll can check this flag and do the
right thing.

Ben.


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