linux-kernel - Re: [PATCH 7/7] perf tools: Set attr.task bit for a tracking event

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Date:	Tue, 23 Dec 2014 13:06:33 +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>,
	Stephane Eranian <eranian@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH 7/7] perf tools: Set attr.task bit for a tracking event

On Mon, Dec 22, 2014 at 11:49:19AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Dec 22, 2014 at 01:44:15PM +0900, Namhyung Kim escreveu:
> > The perf_event_attr.task bit is to track task (fork and exit) events
> > but it missed to be set by perf_evsel__config().  While it was not a
> > problem in practice since setting other bits (comm/mmap) ended up
> > being in same result, it'd be good to set it explicitly anyway.
> 
> I didn't understand, so this isn't strictly needed? I.e. what is the
> point of the attr->task bit then?

Yes, it's not strictly needed for this case.  The attr->task is to
track task related events (fork/exit) only but other meta events like
comm and mmap[2] also needs the task events.  So setting attr->comm
and/or attr->mmap causes the kernel emits the task events anyway.  So
the attr->task is only meaningful when other bits are off but I'd like
to set it for completeness.

Thanks,
Namhyung
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