linux-kernel - Re: [PATCH 35/40] perf record: Synthesize COMM event for a command line workload

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Date:	Wed, 20 May 2015 00:12:45 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 35/40] perf record: Synthesize COMM event for a command
 line workload

On Tue, May 19, 2015 at 11:02:20AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, May 19, 2015 at 04:46:43PM +0900, Namhyung Kim escreveu:
> > On Mon, May 18, 2015 at 09:45:35AM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Mon, May 18, 2015 at 09:30:50AM +0900, Namhyung Kim escreveu:
> > > > When perf creates a new child to profile, the events are enabled on
> > > > exec().  And in this case, it doesn't synthesize any event for the
> > > > child since they'll be generated during exec().  But there's an window
> > > > between the enabling and the event generation.
> > > > 
> > > > It used to be overcome since samples are only in kernel (so we always
> > > > have the map) and the comm is overridden by a later COMM event.
> > > > However it won't work anymore since those samples will go to a missing
> > > > thread now but the COMM event will create a (current) thread.  This
> > > > leads to those early samples (like native_write_msr_safe) not having a
> > > > comm but pid (like ':15328').
> > >  
> > > > So it needs to synthesize COMM event for the child explicitly before
> > > > enabling so that it can have a correct comm.  But at this time, the
> > > > comm will be "perf" since it's not exec-ed yet.
> > > 
> > > This looks reasonable, but I think it probably needs to be done
> > > somewhere in perf_evlist__prepare_workload() or
> > > perf_evlist__start_workload(), as this affects other tools as well, like
> > > 'top', 'trace' and any other that may want to do this start-workload use
> > > case.
> > 
> > Hmm.. I need to look at this again as it only affects on processing
> > indexed data files which used to have a separate missing threads tree.
> 
> Humm, you're thinking about where you managed to reproduce the problem,
> I am thinking outside indexing, etc, i.e. by definition we either enable
> the event before we fork, so that we get the PERF_RECORD_FORK/COMM or we
> synthesize it either from /proc or directly (preferred) if we decide to
> do it after the fork/exec, right?

But as I said before, later COMM event will override thread->comm to a
proper string as long as it can find a matching thread.  So I think it
has no problem in the current code.

In the old version of this patchset (v3), indexing made it impossible
for COMM event to find a matching thread since it used to have a
separate tree for threads that have sampled before any FORK/COMM event
came.  I think it doesn't apply to the current version anymore, I
will check it tomorrow.

Thanks,
Namhyung


> 
> - Arnaldo
> 
> > That's the reason why I didn't put it in a generic place like you
> > said.
> > 
> > However I changed not to use the separate tree - the purpose of the
> > tree was to reduce lock acquisition on thread searching but it already
> > grabs a rwlock with thread refcounting change.
> > 
> > Will check whether this is still needed..
> > 
> > Thanks,
> > Namhyung
> > 
> > 
> > > 
> > > I also wonder if we can't overcome this without using /proc, i.e.
> > > actually moving the "start the workload" to just before the fork, so
> > > that the kernel covers that as well.
> > > 
> > > Or, alternatively, the thread can be created without having to look at
> > > /proc at all, but by directly creating the struct thread, with the
> > > correct COMM, pid, etc, that we know, since we forked it, etc.
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