netdev - Re: napi layer and packet throttling

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Message-ID: <1369399009.3469.273.camel@deadeye.wl.decadent.org.uk>
Date:	Fri, 24 May 2013 13:36:49 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Holger Brunck <holger.brunck@...mile.com>
CC:	<netdev@...r.kernel.org>
Subject: Re: napi layer and packet throttling

On Fri, 2013-05-24 at 09:28 +0200, Holger Brunck wrote:
> Hi Ben,
> 
> On 05/24/2013 12:27 AM, Ben Hutchings wrote:
> > On Thu, 2013-05-23 at 13:52 +0200, Holger Brunck wrote:
> >> 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.
> > [...]
> > 
> > If the RX ring is not cleaned and refilled quickly enough, the network
> > controller will naturally start to drop packets.  It's not something you
> > should do explicitly in the driver.
> > 
> 
> yes. But what if the remaining amount of packets which are getting through the
> napi_poll function into the linux system are still to many and generate
> therefore a to high softirq load on the system which leads to the problems I
> see.

Soft IRQs are then handled in a ksoftirqd thread, which is scheduled
like any other task (though it has high priority).

> Ok I could use a smaller amount of RX ring buffers, but then the system
> would get more intolerant for RX bursts what I don't want. I would like to
> protect the system if someone sends continuously a high packet rate to the
> interface, similar to DoS attacks.

This is not the job of the driver (beyond using NAPI in the first
place).

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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