Re: [PATCH] x86: Get bp from the IRQ regs instead of directly from the CPU

[Ingo Molnar <mingo@elte.hu>]

* Soeren Sandmann <sandmann@daimi.au.dk> wrote:

> Passing 0 for bp causes dump_trace() to get bp directly from the
> hardware register. This leads to the IRQ stack being included in the
> generated call chains, which means the stack looks something like
> this:
> 
> 	[ ip ] [ IRQ stack ] [ rest of stack trace ]
> 
> which is incorrect and confusing to user space.
> 
> Getting bp from the IRQ regs instead makes the tracing start after the
> IRQ stack:
> 
> 	[ ip ] [ rest of stack trace ]
> 
> Signed-off-by: Søren Sandmann Pedersen <sandmann@redhat.com>

Indeed, nice catch!

> ---
>  arch/x86/kernel/cpu/perf_event.c |   10 +++++++++-
>  1 files changed, 9 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index b5801c3..39b1d0c 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -2177,10 +2177,18 @@ static const struct stacktrace_ops backtrace_ops = {
>  static void
>  perf_callchain_kernel(struct pt_regs *regs, struct perf_callchain_entry *entry)
>  {
> +	unsigned long bp;
> +    
>  	callchain_store(entry, PERF_CONTEXT_KERNEL);
>  	callchain_store(entry, regs->ip);
>  
> -	dump_trace(NULL, regs, NULL, 0, &backtrace_ops, entry);
> +#ifdef CONFIG_FRAME_POINTER
> +	bp = regs->bp;
> +#else
> +	bp = 0;
> +#endif
> +	
> +	dump_trace(NULL, regs, NULL, bp, &backtrace_ops, entry);
>  }

Wouldnt it be better to push this logic into dump_trace() itself? That 
way other ways of backtrace generation would be improved as well, not 
just perf events call-chains.

	Ingo