Re: [PATCH] perf tools: Fix use of wrong event when processing exit events

From: Adrian Hunter
Date: Thu Sep 03 2015 - 08:17:13 EST

On 02/09/15 22:55, Arnaldo Carvalho de Melo wrote:
> Em Wed, Sep 02, 2015 at 04:00:08PM +0300, Adrian Hunter escreveu:
>> On 18/08/15 12:07, Adrian Hunter wrote:
>>> In a couple of cases the 'comm' member of 'union event' has
>>> been used instead of the correct member ('fork') when processing
>>> exit events.
>>> In the cases where it has been used incorrectly, only the 'pid'
>>> and 'tid' are affected. The 'pid' value would be correct anyway
>>> because it is in the same position in 'comm' and 'fork' events,
>>> but the 'tid' would have been incorrectly assigned from 'ppid'.
>>> However, for exit events, the kernel puts the current task in
>>> the 'ppid' and 'ttid' which is the same as the exiting task.
>>> That is 'ppid' == 'pid' and if the task is not multi-threaded,
>>> 'pid' == 'tid' i.e. the data goes wrong only when tracing
>>> multi-threaded programs.
>>> It is hard to find an example of how this would produce an
>>> error in practice. There are 3 occurences of the fix:
>>> 1. perf script is only affected if !sample_id_all which only
>>> happens on old kernels
>>> 2. intel_pt is only affected when decoding without timestamps
>>> and would probably still decode correctly - the exit event is
>>> only used to flush out data which anyway gets flushed at the
>>> end of the session
>>> 3. intel_bts also uses the exit event to flush data which
>>> would probably not cause errors as it would get flushed at
>>> the end of the session instead
>>> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>> I think it would be worth picking this one up for 4.3
> I guess I had this picked up, is there anything else from you that is
> outstanding?

Only "perf tools: Add support for PERF_RECORD_SWITCH to Intel PT" but that
is a feature not a bug.

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