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

[Wangnan (F)]

On 2015/11/19 18:23, Ingo Molnar wrote:
> * Wangnan (F) <wangnan0@xxxxxxxxxx> wrote:
>
> > On 2015/11/19 14:37, Ingo Molnar wrote:
> > > * Wangnan (F) <wangnan0@xxxxxxxxxx> 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.
> > Just for curiosity:
> >
> > how the interrupted process continue to execute, when the PC
> > saved in pt_regs still pointed to 'callq' but SP and stack has
> > already changes? Do we fix it in kernel, or by hardware?
> 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.

You mean __intel_pmu_pebs_event(), which generates many perf_events?
Then their output are based on a same user stack, and could be error,
because the instruction has finished, and user stack could be modified.
Right?

Also, why not fixing rsp in kernel if that instruction is a 'callq'?
For avoiding instruction decoding?

Thank you.

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