netdev - napi layer and packet throttling

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Message-ID: <519E030E.5040903@keymile.com>
Date:	Thu, 23 May 2013 13:52:46 +0200
From:	Holger Brunck <holger.brunck@...mile.com>
To:	netdev@...r.kernel.org
Subject: napi layer and packet throttling

Hi all,
I have a general question to the napi layer. I have a board with a MPC8360
processor which uses the ucc_geth.c driver for it's ethernet device. The kernel
version is 3.9. We see in lab tests that the board gets unresponsive if we send
a high packet rate to this ethernet device via an packet generator. In the end
we would like to limit the amount of incoming packets at a very early stage, to
prevent the system of a to high softirq load.

The NAPI layer description I read on:
http://www.linuxfoundation.org/collaborate/workgroups/networking/napi
pointed two main functions out for the napi layer.

a) Interrupt mitigation
Which works perfectly on the ucc_geth.c driver under high load the hard irqs for
the RX packets are disabled and we switch to a polling mode. If the load is more
relaxed we switch back.

b) Packet-Throttling
Here the description says "NAPI-compliant drivers can often cause packets to be
dropped in the network adaptor itself, before the kernel sees them at all."

This is exactly what I need for my usecase. But I don't see any hints how this
can be implemented with the napi layer. So what I need would be something like a
possibility to accept on the eth0 device in timeframe x only packetcount y. If
this packetcount is reached disable hard irq and napi_poll until the timeframe
is expired.

So are there any pointers to other ethernet drivers doing something like this?
Do I misunderstand the packet throttling purpose of the NAPI layer completely?

Any help is appreciated.

Regards
Holger

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