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Message-ID: <20151201161125.GA2441@gmail.com>
Date: Tue, 1 Dec 2015 17:11:25 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Wangnan (F)" <wangnan0@...wei.com>, Jiri Olsa <jolsa@...nel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
David Ahern <dsahern@...il.com>,
Milian Wolff <milian.wolff@...b.com>,
linux-kernel@...r.kernel.org, pi3orama <pi3orama@....com>,
lizefan 00213767 <lizefan@...wei.com>
Subject: Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling
taken at 'callq' instruction
* Peter Zijlstra <peterz@...radead.org> wrote:
> > so I think the problem here is that the RSP does not match up to the RIP. We
> > can either pass along the original RIP+RSP, or the fixed up one - but what we
> > do currently is that we pass along only half of it - which corrupts dwarf
> > unwinding state that doesn't tolerate such errors.
>
> Still not sure what that gets you. Then you get a sample at a known wrong
> location, why would you want that?
Well, we'd at least get a valid call trace - which the 'mixed' one isn't? I.e.
this only matters with --call-graph.
But yeah, with my suggestion we'd essentially fall back from cycles:pp to
cycles:p, ideally we'd want to have real_rsp. Does the hardware provide that?
User-space cannot compute that reliably I think, what if the 'real' instruction
was manipulating RSP in more complex ways than doing a CALL?
Thanks,
Ingo
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