From: takahiro.akashi@linaro.org (AKASHI Takahiro)
To: linux-arm-kernel@lists.infradead.org
Subject: [RFC 2/3] arm64: refactor save_stack_trace()
Date: Wed, 15 Jul 2015 20:41:34 +0900 [thread overview]
Message-ID: <55A646EE.6030402@linaro.org> (raw)
In-Reply-To: <20150714225105.6c1e4f15@gandalf.local.home>
Steve,
On 07/15/2015 11:51 AM, Steven Rostedt wrote:
> On Wed, 15 Jul 2015 09:20:42 +0900
> AKASHI Takahiro <takahiro.akashi@linaro.org> wrote:
>
>> On 07/14/2015 10:31 PM, Steven Rostedt wrote:
>>> On Tue, 14 Jul 2015 21:47:10 +0900
>>> Jungseok Lee <jungseoklee85@gmail.com> wrote:
>>>
>>>> Is the below example an unexpected result?
>>>> Entry 17 and 18 are ftrace_call and ftrace_ops_no_ops, respectively.
>>
>> [snip]
>>
>>> Note, function tracing does not disable interrupts. This looks to be
>>> that an interrupt came in while __aloc_skb() was being traced.
>>
>> Yeah, I think so, too. But if my insight is correct, it's not __alloc_skb()
>> but one of functions that it calls. As I said in the commit log message
>> of patch[1/3], the exact traced function will not be listed by
not patch[1/3], but patch[3/3]
>> save_stack_trace() because we don't create a stack frame at mcount().
>> I think this is a flaw in the current implementation (on x86).
>>
>> what do you think, Steve?
>>
>
> mcount (well ftrace_call actually) does indeed create a stack frame for
> itself *and* for what called it. At least on x86_64. See mcount_64.S.
>
> With -pg -mfentry, it creates a stack frame. Without -mfentry, mcount
> is called after the current function's frame is made so we don't need
> to do much.
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
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?
Thanks,
-Takahiro AKASHI
> Here's what the -mfentry version does:
>
> pushq %rbp
> pushq 8*2(%rsp) /* this is the parent pointer */
> pushq %rbp
> movq %rsp, %rbp
> pushq 8*3(%rsp) /* Return address to ftrace_call */
> pushq %rbp
> movq %rsp, %rbp
>
>
> Thus the stack looks like this:
>
> <---+
> | | |
> +------------------------------+ |
> | return address for func | |
> | return address for func_call | |
> | original %rbp | |
> +------------------------------+ |
> | return address for func | |
> | ptr to parent frame (%rbp) | ----+
> +------------------------------| <-----+
> | return address for func_call | |
> | ptr to next frame (%rbp) | ------+
> +------------------------------+ <---+
> |
> |
> Current %rbp points to func_call frame -----+
>
> The first box isn't used as a frame, but is used by ftrace_call to save
> information to restore everything properly.
>
> Thus, __alloc_skb() is what is currently being traced.
>
>
> -- Steve
>
next prev parent reply other threads:[~2015-07-15 11:41 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-13 5:29 [RFC 0/3] arm64: ftrace: fix incorrect output from stack tracer AKASHI Takahiro
2015-07-13 5:29 ` [RFC 1/3] ftrace: adjust a function's pc to search for in check_stack() for arm64 AKASHI Takahiro
2015-07-13 15:24 ` Steven Rostedt
2015-07-15 0:22 ` AKASHI Takahiro
2015-07-13 5:29 ` [RFC 2/3] arm64: refactor save_stack_trace() AKASHI Takahiro
2015-07-14 12:47 ` Jungseok Lee
2015-07-14 13:31 ` Steven Rostedt
2015-07-15 0:20 ` AKASHI Takahiro
2015-07-15 2:51 ` Steven Rostedt
2015-07-15 11:41 ` AKASHI Takahiro [this message]
2015-07-15 14:55 ` Steven Rostedt
2015-07-15 16:13 ` Steven Rostedt
2015-07-16 0:27 ` AKASHI Takahiro
2015-07-16 1:08 ` AKASHI Takahiro
2015-07-16 1:38 ` Steven Rostedt
2015-07-17 10:46 ` Will Deacon
2015-07-16 13:29 ` Jungseok Lee
2015-07-16 13:54 ` Jungseok Lee
2015-07-16 14:24 ` Steven Rostedt
2015-07-16 15:01 ` Jungseok Lee
2015-07-16 15:31 ` Steven Rostedt
2015-07-16 15:52 ` Jungseok Lee
2015-07-16 20:22 ` Steven Rostedt
2015-07-17 2:49 ` AKASHI Takahiro
2015-07-17 3:21 ` Steven Rostedt
2015-07-16 16:16 ` Steven Rostedt
2015-07-17 12:40 ` Mark Rutland
2015-07-17 12:51 ` Steven Rostedt
2015-07-17 13:00 ` Steven Rostedt
2015-07-17 14:28 ` Jungseok Lee
2015-07-17 14:41 ` Steven Rostedt
2015-07-17 14:59 ` Jungseok Lee
2015-07-17 15:34 ` Jungseok Lee
2015-07-17 16:01 ` Steven Rostedt
2015-07-20 16:20 ` Will Deacon
2015-07-20 23:53 ` AKASHI Takahiro
2015-07-21 10:26 ` AKASHI Takahiro
2015-07-21 14:34 ` Jungseok Lee
2015-08-03 9:09 ` Will Deacon
2015-08-03 14:01 ` Steven Rostedt
2015-08-03 14:04 ` Will Deacon
2015-08-03 16:30 ` Jungseok Lee
2015-08-03 16:57 ` Steven Rostedt
2015-08-03 17:22 ` Jungseok Lee
2015-08-03 17:32 ` Steven Rostedt
2015-08-04 7:41 ` AKASHI Takahiro
2015-07-17 2:04 ` AKASHI Takahiro
2015-07-17 14:38 ` Jungseok Lee
2015-07-16 14:28 ` Mark Rutland
2015-07-16 14:34 ` Steven Rostedt
2015-07-17 2:09 ` AKASHI Takahiro
2015-07-13 5:29 ` [RFC 3/3] arm64: ftrace: mcount() should not create a stack frame AKASHI Takahiro
2015-07-13 15:01 ` [RFC 0/3] arm64: ftrace: fix incorrect output from stack tracer Jungseok Lee
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