linux-kernel - Re: [PATCH v5 0/6] arm64: ftrace: fix incorrect output from stack tracer

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Date:	Tue, 10 Nov 2015 11:58:24 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Jungseok Lee <jungseoklee85@...il.com>
Cc:	catalin.marinas@....com, will.deacon@....com, rostedt@...dmis.org,
	broonie@...nel.org, david.griego@...aro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/6] arm64: ftrace: fix incorrect output from stack
 tracer

On 11/09/2015 11:24 PM, Jungseok Lee wrote:
> On Nov 6, 2015, at 3:44 PM, AKASHI Takahiro wrote:
>
> Hi Akashi,
>
>> This is the fifth patch series for fixing stack tracer on arm64.
>> The original issue was reported by Jungseok[1], and then I found more
>> issues[2].
>>
>> We don't have to care about the original issue because the root cause
>> (patch "ARM64: unwind: Fix PC calculation") has been reverted in v4.3.
>>
>> I address here all the issues and implement fixes described in [2] except
>> for interrupt-triggered problems(II-3) and leaf function(II-5).  Recent
>> discussions[3] about introducing a dedicated interrupt stack suggests that
>> we may avoid walking through from an interrupt stack to a process stack.
>> (So interrupt-stack patch is a prerequisite.)
>>
>> Basically,
>> patch1 is a proactive improvement of function_graph tracer.
>> patch2 corresponds to II-4(functions under function_graph tracer).
>> patch3, 4 and 5 correspond to II-1(slurping stack) and II-2(differences
>> between x86 and arm64).
>> patch6 is a function prologue analyzer test. This won't attest
>> the correctness of the functionality, but it can suggest that all
>> the traced functions are treated properly by this function.
>> (Please note that patch3 has already been queued in Steven's for-next.)
>>
>> I tested the code with v4.3 + Jungseok's patch v5[4].
>
> I've played this series with IRQ stack patch and it works well at least
> on my system! In addition to this condition, I've run these changes without
> IRQ stack since it is in progress. I could observe a single strange behavior,
> minus stack size around elX_irq. Am I missing something?

You saw the result like:
  ...
  13)     4336      64   gic_handle_irq+0x5c/0xa4
  14)     4272     576   el1_irq+0x68/0xd8
  15)     3696    -160   smc_hardware_send_pkt+0x278/0x42c

This is the most difficult problem that I mentioned in II-3 of [1] and tried to fix.
For example, smc_hardware_send_pkt is NOT the function interrupted, but
_raw_spin_unlock_irqstore which is called at '+0x278/0x42c' is.
Giving a *perfect* solution against it is quite tough (and complicated).
Since you have introduced interrupt stack and even on x86 an interrupt stack is
not supported, I removed related patches.

-Takahiro AKASHI

> My reproduction scenario is simple.
>
>    $ sudo echo 1 > /proc/sys/kernel/stack_trace_enabled
>    $ sudo echo function_graph > /sys/kernel/debug/tracing/current_tracer
>    $ [ Run any workload ]
>    $ sudo cat /sys/kernel/debug/tracing/stack_trace
>
> Best Regards
> Jungseok Lee
>
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