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

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Date:	Thu, 19 Nov 2015 12:23:15 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.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

On Thu, Nov 19, 2015 at 11:23:00AM +0100, Ingo Molnar wrote:
> PEBS is an asynchronous hardware tracing mechanism, when batched PEBS is used it 
> might not even result in any interruption of execution. The 'pt_regs' does not 
> necessarily correspond to an interrupted, restartable context - we take the RIP 
> from the PEBS machinery and also use LBR and disassembly to determine the previous 
> instruction, before reporting it to user-space.

Note that modern PEBS hardware (hsw+) does the rollback in hardware.
Prior to that we indeed to it manually using the LBR.

As to pt_regs, we construct a franken pt_regs based on the actual PEBS
buffer overflow PMI and bits from the PEBS record (which also includes
some register state). See
arch/x86/kernel/cpu/perf_event_intel_ds.c:setup_pebs_sample_data().

We always copy the flags, ip, bp and sp from the PEBS record into the
interrupt pt_regs.

And note that the PEBS record is constructed at instruction retirement,
so it shows the state _after_ the instruction, with exception of the
(hsw+) real_ip field.

So the unwinder will have to be taught that if the IP points at a stack
altering instruction (call, push, etc.) it will have to 'undo' the
effects on the actual stack (I appreciate this might be 'interesting'
for things like: pop, ret, etc.).

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