linux-kernel - Re: [RFC PATCH net-next v8 07/14] page

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Date: Wed, 1 May 2024 15:58:59 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Jens Axboe <axboe@...nel.dk>, Mina Almasry <almasrymina@...gle.com>
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Subject: Re: [RFC PATCH net-next v8 07/14] page_pool: devmem support

On 30/04/2024 20.55, Jens Axboe wrote:
> On 4/30/24 12:29 PM, Mina Almasry wrote:
>> On Tue, Apr 30, 2024 at 6:46?AM Jens Axboe<axboe@...nel.dk>  wrote:
[...]
>>> In general, attempting to hide overhead behind config options is always
>>> a losing proposition. It merely serves to say "look, if these things
>>> aren't enabled, the overhead isn't there", while distros blindly enable
>>> pretty much everything and then you're back where you started.
>>>
>> The history there is that this check adds 1 cycle regression to the
>> page_pool fast path benchmark. The regression last I measured is 8->9
>> cycles, so in % wise it's a quite significant 12.5% (more details in
>> the cover letter[1]). I doubt I can do much better than that to be
>> honest.
>
> I'm all for cycle counting, and do it myself too, but is that even
> measurable in anything that isn't a super targeted microbenchmark? Or
> even in that?

The reason for page_pool fast path being critical is that it is used for 
the XDP_DROP use-case.
E.g on Mellanox mlx5 driver we see 24 Mpps XDP_DROP, which is approx 42 
nanosec per packet. Adding 9 nanosec will reduce this to 19.6 Mpps.

   1/(42+9)*10^9 = 19607843

--Jesper

p.s. Upstreaming my PP microbenchmark[1] is still at the bottom of my 
todo-list.
  [1] 
https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_simple.c