netdev - Re: [PATCH net-next 1/4] tsnep: Throttle interrupts

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Message-ID: <784e1ad7-0110-43f9-342c-abe513123f1a@engleder-embedded.com>
Date:   Fri, 18 Nov 2022 06:50:47 +0100
From:   Gerhard Engleder <gerhard@...leder-embedded.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        edumazet@...gle.com, pabeni@...hat.com
Subject: Re: [PATCH net-next 1/4] tsnep: Throttle interrupts

On 17.11.22 21:33, Andrew Lunn wrote:
> On Thu, Nov 17, 2022 at 09:14:37PM +0100, Gerhard Engleder wrote:
>> Without interrupt throttling, iperf server mode generates a CPU load of
>> 100% (A53 1.2GHz). Also the throughput suffers with less than 900Mbit/s
>> on a 1Gbit/s link. The reason is a high interrupt load with interrupts
>> every ~20us.
> 
> Not my area of expertise, but is NAPI working correctly? It should be
> that you disable interrupts while NAPI is polling, and only re-enable
> interrupts when polling has stopped. If you are receiving at near line
> rate at 100% load, i would of thought that NAPI would be polling most
> of the time and interrupts would be mostly disabled?

I disable and re-enable interrupts in the driver, but I don't know
exactly under which conditions interrupts will be disabled mostly. At
least during my XDP tests with minimum size packets the interrupt rate
dropped significantly. Here the CPU had also 100% load but packet rate
was much higher with ~880000 pkt/s.

I compared the performance and the interrupt rate with the macb driver
on the same CPU and the results were similar.

> Interrupt coalescence makes a lot of sense thought, so the patch
> itself is useful.

Thanks!

Gerhard