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

From: Ingo Molnar
Date: Thu Nov 19 2015 - 05:23:14 EST

* 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.


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