Re: [PATCH] Perf: bug fix: distinguish between rename and exec

[Arnaldo Carvalho de Melo]

Em Fri, Mar 02, 2012 at 02:44:37PM +0100, Stephane Eranian escreveu:
> Did we come to an agreement on this problem?
> 
> Seems like we do want to distinguish exec from renames, as perf
> David's example with top.
> In Luigi's patch, the distinction is made via the header->misc
> bitmask. Now, I see people have
> proposed a new record type: PERF_RECORD_COMM vs. PERF_RECORD_EXEC. This would
> also work but it would require a lot more changes in the tool, i.e.,

He is working on it, already submitted a RFC and will follow up on lkml
soon,

- Arnaldo

> you have to process a new
> record type. What's wrong with the header->misc approach?
> 
> 
> On Wed, Feb 15, 2012 at 6:47 PM, Arnaldo Carvalho de Melo
> <acme@xxxxxxxxxxxxxxxxxx> wrote:
> > Em Wed, Feb 15, 2012 at 09:07:47AM -0800, Luigi Semenzato escreveu:
> >> On Wed, Feb 15, 2012 at 5:47 AM, Arnaldo Carvalho de Melo
> >> <acme@xxxxxxxxxxxxxxxxxx> wrote:
> >> > Em Wed, Feb 15, 2012 at 01:48:33PM +0100, Peter Zijlstra escreveu:
> >> >> I really dislike changing generic code purely for the purpose of
> >> >> instrumentation like this. Better to pull perf_event_comm() out of here
> >> >> if you want to change semantics.
> >> >>
> >> >> Personally I couldn't care less about renames, I think they're a waste
> >> >> of time, so I'm ok with the simple patch moving the perf_event_comm()
> >> >> into setup_new_exec() and possibly renaming it to perf_event_exec().
> >> >>
> >> >> Acme, do you care about renames?
> >> >
> >> > I like your idea of keeping the semantics of PERF_RECORD_COMM and
> >> > introducing a PERF_RECORD_EXEC, just have to think about how to handle
> >> > that in a way that the tools detect that we have PERF_RECORD_EXEC...
> >>
> >> I considered this but I don't know how important it is to be backward
> >> compatible. ÂAdding a new record type makes old "perf report" fail to
> >> parse new perf.data files. Â(Unless we pad the new record to a
> >
> > Hey, old perf record would still see the PERF_RECORD_COMM, i.e. we would
> > continue asking for PERF_RECORD_COMM in new versions. Together with
> > PERF_RECORD_EXEC.
> >
> >> multiple of 8 bytes, but I don't think we want to go down that path).
> >>
> >> If looking forward is more important, I agree a new new record type is
> >> best. ÂWe might want to consider adding a PERF_RECORD_RENAME for
> >
> > PERF_RECORD_COMM is good enough, well, it always was confusing for most
> > people that asked "hey, that means an EXEC, right?"
> >
> > First thing pople think is "hey, this is when it sets the thread COMM,
> > right?"
> >
> >> renames, and leaving the COMM record to its historical meaning (exec),
> >> possibly renaming it to PERF_RECORD_EXEC for clarity. ÂAnd yes, the
> >> perf instrumentation should not be in set_task_comm(), that's why the
> >> bug exists in the first place.
> >>
> >> We might also want to change the parsing of perf.data so that in the
> >> future it is more tolerant of new record types.
> >
> > Yes, what is the behaviour now? Lemme see... Well, difficult, I'm barely
> > reading this, just after magnifying it, dilated pupils two hours ago,
> > grrr
> >
> > WiÄl check later, but IIRC it just warns and skips the record, right?
> >
> >> > Humm, will be yet another fallback for setting an perf_event_attr bit,
> >> > just like with .sample_id_all and .exclude_{guest,host}...
> >> >
> >> > That together with the per class errnos + __strerror() method will allow
> >> > to move all the event creation finally to perf_evlist__open() where all
> >> > this gets nicely hidden away from poor tools.
> >> >
> >> > We can then even have an ui__evlist_perror() method that does all the
> >> > ui__warning calls, etc.
> >> >
> >> > So, yes, from a tooling perspective, I want to be notified of renames
> >> > and being able to stop relying on PERF_RECORD_COMM to call
> >> > map_groups__flush and instead do it at PERF_RECORD_EXEC seems a
> >> > bonus.
> >> >
> >> > - Arnaldo
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