linux-kernel - Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling taken at 'callq' instruction

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Message-ID: <20151119063709.GA14852@gmail.com>
Date:	Thu, 19 Nov 2015 07:37:10 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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


* Wangnan (F) <wangnan0@...wei.com> wrote:

> >perf cmdline is
> >
> ># ./pref record  -g -F 9 --call-graph dwarf ./test_dwarf_unwind
> >
> >Use default events, precise_ip == 2 so uses PEBS.
> >
> 
> Testetd 'cycles', 'cycles:p' and 'cycles:pp'. Only 'cycles:pp' captures
> sample at callq. So maybe a PEBS problem?

Well, that's how our PEBS sampling works: we roll back the instruction pointer to 
point at the instruction generating the sample. The state itself is 
post-instruction.

So dwarf unwind has to take this into account. Should the kernel pass in the 
original (uncorrected) RIP as well, perhaps?

Thanks,

	Ingo
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