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

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Date:	Fri, 23 Oct 2009 12:50:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Soeren Sandmann <sandmann@...mi.au.dk>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] x86: Get bp from the IRQ regs instead of directly from
	the CPU


* Soeren Sandmann <sandmann@...mi.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: SÃ¸ren Sandmann Pedersen <sandmann@...hat.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
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