linux-kernel - Re: [RFC 2/3] arm64: refactor save_stack

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Date:	Wed, 15 Jul 2015 10:55:36 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:	Jungseok Lee <jungseoklee85@...il.com>, catalin.marinas@....com,
	will.deacon@....com, olof@...om.net, broonie@...nel.org,
	david.griego@...aro.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/3] arm64: refactor save_stack_trace()

On Wed, 15 Jul 2015 20:41:34 +0900
AKASHI Takahiro <takahiro.akashi@...aro.org> wrote:


> Thank you for the explanation. But what I don't really understand here
> is why we need to add the "current function" to the stack dump list
> returned by save_stack_trace():
> 
> In check_stack(),
>  >        /*
>  >         * Add the passed in ip from the function tracer.
>  >         * Searching for this on the stack will skip over
>  >         * most of the overhead from the stack tracer itself.
>  >         */
>  >        stack_dump_trace[0] = ip;
>  >        max_stack_trace.nr_entries++;
> 
> I think that "ip" here is the "return address for func" in your

Ah, you are correct (for fentry).

> ascii art, and it should be already in the list if a frame is made
> by mcount (or func_call).
> 
> In fact, stack tracer on arm64 works OK even without the patch[3/3]
> if the code quoted above is commented out.
> Even on x86, the code is conditional and not activated if the kernel
> is compiled without -mfentry before the following commit:
>      commit 4df297129f62 ("tracing: Remove most or all of stack tracer stack size from stack_max_size")
> 
> So what do I misunderstand here?
> 

Hmm, I haven't touched the stack trace code in a while. With the added
stack frame for fentry, the hacks there are probably not needed.

I'll take a look at it and try to clean up the code.

Thanks,

-- Steve
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