netdev - Re: [PATCH v5] net: ethernet: add driver for Aurora VLSI NB8800 Ethernet controller

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Date:	Wed, 11 Nov 2015 13:04:07 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	David Miller <davem@...emloft.net>
Cc:	romieu@...zoreil.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, slash.tmp@...e.fr
Subject: Re: [PATCH v5] net: ethernet: add driver for Aurora VLSI NB8800 Ethernet controller

Måns Rullgård <mans@...sr.com> writes:

> David Miller <davem@...emloft.net> writes:
>
>> From: Måns Rullgård <mans@...sr.com>
>> Date: Wed, 11 Nov 2015 00:40:09 +0000
>>
>>> When the DMA complete interrupt arrives, the next chain should be
>>> kicked off as quickly as possible, and I don't see why that would
>>> benefit from being done in napi context.
>>
>> NAPI isn't about low latency, it's about fairness and interrupt
>> mitigation.
>>
>> You probably don't even realize that all of the TX SKB freeing you do
>> in the hardware interrupt handler end up being actually processed by a
>> scheduled software interrupt anyways.
>>
>> So you are gaining almost nothing by not doing TX completion in NAPI
>> context, whereas by doing so you would be gaining a lot including
>> more simplified locking or even the ability to do no locking at all.
>
> TX completion is separate from restarting the DMA, and moving that to
> NAPI may well be a good idea.  Should I simply napi_schedule() if the
> hardware indicates TX is complete and do the cleanup in the NAPI poll
> function?

I tried that, and throughput (as measured by iperf3) dropped by 2%.
Maybe I did something wrong.

-- 
Måns Rullgård
mans@...sr.com
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