Date: Tue, 1 Dec 2015 17:11:25 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Wangnan (F)" <wangnan0@huawei.com>, Jiri Olsa <jolsa@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        David Ahern <dsahern@gmail.com>, Milian Wolff <milian.wolff@kdab.com>,
        linux-kernel@vger.kernel.org, pi3orama <pi3orama@163.com>,
        lizefan 00213767 <lizefan@huawei.com>
Subject: Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling
 taken at 'callq' instruction
Message-ID: <20151201161125.GA2441@gmail.com>
References: <564C3A0E.3030502@huawei.com>
 <564C3BAA.4040806@huawei.com>
 <20151119063709.GA14852@gmail.com>
 <564D6FF9.3030105@huawei.com>
 <20151119102300.GA2830@gmail.com>
 <20151119112315.GL3816@twins.programming.kicks-ass.net>
 <20151127083811.GA26257@gmail.com>
 <20151130092843.GF17308@twins.programming.kicks-ass.net>
 <20151201072826.GB28270@gmail.com>
 <20151201083845.GI3816@twins.programming.kicks-ass.net>
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* Peter Zijlstra <peterz@infradead.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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