linux-kernel - Re: LBR callchains from tracepoints

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Message-ID: <20160426172629.GA42777@ast-mbp.thefacebook.com>
Date:	Tue, 26 Apr 2016 10:26:32 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	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 Tue, Apr 26, 2016 at 06:38:28PM +0200, Peter Zijlstra wrote:
> 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.

btw we're thinking to add support for lbr to bpf, so that from the program
we can get accurate and fast stacks. That's especially important for user
space stacks. No clear idea how to do it yet, but it would be really useful.