netdev - Re: [PATCH net-next 1/3] perf, events: add non-linear data support for raw records

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Date:	Wed, 13 Jul 2016 18:53:27 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	davem@...emloft.net, alexei.starovoitov@...il.com, tgraf@...g.ch,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] perf, events: add non-linear data support
 for raw records

On 07/13/2016 06:40 PM, Peter Zijlstra wrote:
> On Wed, Jul 13, 2016 at 04:08:55PM +0200, Daniel Borkmann wrote:
>> On 07/13/2016 03:42 PM, Peter Zijlstra wrote:
>>>
>>> Ok so the nonlinear thing was it doing _two_ copies, one the regular
>>> __output_copy() on raw->data and second the optional fragment thingy
>>> using __output_custom().
>>>
>>> Would something like this work instead?
>>>
>>> It does the nonlinear thing and the custom copy function thing but
>>> allows more than 2 fragments and allows each fragment to have a custom
>>> copy.
>>>
>>> It doesn't look obviously more expensive; it has the one ->copy branch
>>> extra, but then it doesn't recompute the sizes.
>>
>> Yes, that would work as well on a quick glance with diff just a bit
>> bigger, but more generic this way. Do you want me to adapt this into
>> the first patch?
>
> Please.
>
>> One question below:
>>
>
>>> -			u64 zero = 0;
>
>>> -			if (real_size - raw_size)
>>> -				__output_copy(handle, &zero, real_size - raw_size);
>
>> We still need the zero padding here from above with the computed
>> raw->size, right?
>
> Ah, yes, we need some __output*() in order to advance the handle offset.
> We don't _need_ to copy the 0s, but I doubt __output_skip() is much
> cheaper for these 1-3 bytes worth of data; we've already touched that
> line anyway.

Okay, thanks for your input! I'll respin then.