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SubjectRe: [PATCH net-next] perf/core: Put size of a sample at the end of it by PERF_SAMPLE_TAILSIZE
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:
>
> head
> |
> V
> +--+---+-------+----------+------+---+
> |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.
>
> [1] http://lkml.kernel.org/g/1449063499-236703-1-git-send-email-wangnan0@huawei.com
>
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: David Ahern <dsahern@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Yunlong Song <yunlong.song@huawei.com>
> Cc: David S. Miller <davem@davemloft.net>
[...]

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

Cheers,
Daniel

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