Date: Tue, 1 Dec 2015 09:38:45 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.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: <20151201083845.GI3816@twins.programming.kicks-ass.net>
References: <20151118082033.GA24726@gmail.com>
 <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>
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On Tue, Dec 01, 2015 at 08:28:26AM +0100, Ingo Molnar wrote:
> > Not sure what you're getting at here. We don't need the uncorrected
> > instruction.
> 
> Well, we need it for stack unwinding, as you point it out:
> 
> > But the problem here is that we rewind the instruction stream, but not
> > the stack. And the stack unwinder is (obviously) interested in the stack
> > state.
> 
> Unwinding the stack state would fix it as well - but an equivalent solution would 
> be to pass along the original RIP would fix it as well: we'd have a 
> self-consistent pair of RIP/RSP.
> 
> Especially since unwinding the RSP is probably hard:
> 
> > I'm not sure we want (or need) to go undo the specific instruction's
> > stack effect in-kernel. If the !DWARF unwinders are similarly confused
> > we might need to put it in kernel (expensive *groan*). If its only the
> > DWARF muck then its something that can be done in userspace just
> > fine, although we might need to copy slightly more of the stack than SP
> > is pointing at, such that we can undo RET/POP etc. which would have data
> > beyond the head of stack.
> > 
> > The easiest solution might be to figure out the biggest stack offset for
> > any instruction and always capture that much over the head of stack.
> 
> 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?
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