linux-kernel - Re: LBR callchains from tracepoints

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Date:	Tue, 26 Apr 2016 18:38:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	David Ahern <dsahern@...il.com>,
	Milian Wolff <milian.wolff@...b.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: LBR callchains from tracepoints

On Mon, Apr 25, 2016 at 10:24:31PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Apr 25, 2016 at 10:03:58PM -0300, Arnaldo Carvalho de Melo escreveu:
> > I now need to continue investigation why this doesn't seem to work from
> > tracepoints...
> 
> Bummer, the changeset (at the end of this message) hasn't any
> explanation, is this really impossible? I.e. LBR callstacks from
> tracepoints? Even if we set perf_event_attr.exclude_callchain_kernel?

Could maybe be done, but its tricky to implement as the LBR is managed
by the hardware PMU and tracepoints are a software PMU, so we need to
then somehow frob with cross-pmu resources, in a very arch specific way.
And programmability of the hardware PMU will then depend on events
outside of it.

All rather icky.