linux-kernel - Re: [RFC PATCH 0/5] Make eBPF programs output data to perf event

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Date:	Thu, 2 Jul 2015 11:31:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"Wangnan (F)" <wangnan0@...wei.com>, He Kuang <hekuang@...wei.com>,
	rostedt@...dmis.org, masami.hiramatsu.pt@...achi.com,
	mingo@...hat.com, acme@...hat.com, jolsa@...nel.org,
	namhyung@...nel.org, linux-kernel@...r.kernel.org,
	pi3orama <pi3orama@....com>
Subject: Re: [RFC PATCH 0/5] Make eBPF programs output data to perf event

On Wed, Jul 01, 2015 at 07:48:25PM -0700, Alexei Starovoitov wrote:
> On 7/1/15 4:58 AM, Peter Zijlstra wrote:
> >
> >But why create a separate trace buffer, it should go into the regular
> >perf buffer.
> 
> +1
> 
> I think
> +static char __percpu *perf_extra_trace_buf[PERF_NR_CONTEXTS];
> is redundant.
> It adds quite a bit of unnecessary complexity to the whole patch set.

Ah, that is what he does. I through he was creating a trace buffer, as
in the stuff ftrace uses, to output stuff concurrently to the regular
perf buffer, which would be insane ;-)

So you need these temporary buffers if you cannot tell a-priory how much
data is going to get written. The perf buffer can only deal with fixed
size events, you request a buffer of @size, you write into it, you
commit.

So if there's variable sized data, you first need to generate it in
order to tell how long it is, so that you can reserve your record and
copy it in. Sucks, but that's the way it is.
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