Re: [PATCH net-next] perf/core: Put size of a sample at the end of it by PERF_SAMPLE_TAILSIZE

From: Daniel Borkmann
Date: Mon Jan 11 2016 - 12:05:25 EST

Hi Wang,

On 01/11/2016 02:24 PM, Wang Nan wrote:
This patch introduces a PERF_SAMPLE_TAILSIZE flag which allows a size
field attached at the end of a sample. The idea comes from [1] that,
with tie size at tail of an event, it is possible for user program who
read from the ring buffer parse events backward.

For example:

|E6|...| B 8| C 11| D 7|E..|

In this case, from the 'head' pointer provided by kernel, user program
can first see '6' by (*(head - sizeof(u64))), then it can get the start
pointer of record 'E', then it can read size and find start position
of record D, C, B in similar way.

The implementation is easy: adding a PERF_SAMPLE_TAILSIZE flag, makes
perf_output_sample() output size at the end of a sample.

Following things are done for ensure the ring buffer is safe for
backward parsing:

- Don't allow two events with different PERF_SAMPLE_TAILSIZE setting
set their output to each other;

- For non-sample events, also output tailsize if required.

This patch has a limitation for perf:

Before reading such ring buffer, perf must ensure all events which may
output to it is already stopped, so the 'head' pointer it get is the
end of the last record.


Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Yunlong Song <>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>

As this seems unrelated to networking, net-next tree seems not really appropriate?