linux-kernel - 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 18:40:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Borkmann <daniel@...earbox.net>
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 Wed, Jul 13, 2016 at 04:08:55PM +0200, Daniel Borkmann wrote:
> Hi Peter,
> 
> 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.