netdev - Re: NIU driver: Sun x8 Express Quad Gigabit Ethernet Adapter (perf + regression IRQs)

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Message-Id: <20081119.151138.93409143.davem@davemloft.net>
Date:	Wed, 19 Nov 2008 15:11:38 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	hawk@...u.dk
Cc:	jdb@...x.dk, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: NIU driver: Sun x8 Express Quad Gigabit Ethernet Adapter (perf
 + regression IRQs)

From: Jesper Dangaard Brouer <hawk@...u.dk>
Date: Wed, 19 Nov 2008 23:58:12 +0100 (CET)

> Well that was not the real cause of the performance loss.  Because
> on kernel 2.6.27 I get really good performance (900-1200kpps)
> compared to 2.6.28 (git net-2.6).
>
> The cause of this problem (tracked down together with Robert Olsson)
> is that on 2.6.28 I have a lot less IRQs available.  It seems max 34
> IRQs.
>
> Due the reduced number of IRQs the NIU driver cannot get enough IRQs
> to the interfaces, and starts to use "IO-APIC" based IRQs.

This is almost certainly related to the driver unload bug.

I know you ran into unbuildable/unbootable kernels during a bisect,
but you really need to track down this regression.

There were a lot of IRQ changes, especially on x86.  The sequence is
something like:

1) dyn irqs
2) APIC/IO_APIC handling integration
3) by-hand REVERT of dyn irqs, it was done by hand in order to not
   lose the #2 changes
4) interrupt remapping support

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