netdev - Re: Optimizing instruction-cache, more packets at each stage

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Message-ID: <CALx6S37+9tXjrzuhgbsUDPOBEY8uS9Nitk4jE2z0ZZ+ufE-U+w@mail.gmail.com>
Date:	Mon, 18 Jan 2016 09:36:32 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Alexander Duyck <alexander.duyck@...il.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Daniel Borkmann <borkmann@...earbox.net>, marek@...udflare.com,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>,
	Paolo Abeni <pabeni@...hat.com>,
	John Fastabend <john.r.fastabend@...el.com>
Subject: Re: Optimizing instruction-cache, more packets at each stage

On Mon, Jan 18, 2016 at 2:27 AM, Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
> On Fri, 15 Jan 2016 15:47:21 -0500 (EST)
> David Miller <davem@...emloft.net> wrote:
>
>> From: Jesper Dangaard Brouer <brouer@...hat.com>
>> Date: Fri, 15 Jan 2016 14:22:23 +0100
>>
>> > This was only at the driver level.  I also would like some API towards
>> > the stack.  Maybe we could simple pass a skb-list?
>>
>> Datastructures are everything so maybe we can create some kind of SKB
>> bundle abstractions.  Whether it's a lockless array or a linked list
>> behind it doesn't really matter.
>>
>> We could have two categories: Related and Unrelated.
>>
>> If you think about GRO and routing keys you might see what I am getting
>> at. :-)
>
> Yes, I think I get it.  I like the idea of Related and Unrelated.
> We already have GRO packets which is in the "Related" category/type.
>
> I'm wondering about the API between driver and "GRO-layer" (calling
> napi_gro_receive):
>
> Down in the driver layer (RX), I think it is too early to categorize
> Related/Unrelated SKB's, because we want to delay touching packet-data
> as long as possible (waiting for the prefetcher to get data into
> cache).
>
Does DDIO address this?

> We could keep the napi_gro_receive() call.  But in-order to save
> icache, then the driver could just create it's own simple loop around
> napi_gro_receive().  This loop's icache and extra function call per
> packet would cost something.
>
> The down side is: The GRO layer will have no-idea how many "more"
> packets are coming.  Thus, it depends on a "flush" API, which for
> "xmit_more" didn't work out that well.
>
> The NAPI drivers actually already have a flush API (calling
> napi_complete_done()), BUT it does not always get invoked, e.g. if the
> driver have more work to do, and want to keep polling.
>  I'm not sure we want to delay "flushing" packets queued in the GRO
> layer for this long(?).
>
>
> The simplest solution to get around this (flush and driver loop
> complexity), would be to create a SKB-list down in the driver, and
> call napi_gro_receive() with this list.  Simply extending napi_gro_receive()
> with a SKB list loop.
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   Author of http://www.iptv-analyzer.org
>   LinkedIn: http://www.linkedin.com/in/brouer