Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1182524 > unrolled thread

[RFC 1/3] ftrace: adjust a function's pc to search for in check_stack() for arm64

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  [RFC 1/3] ftrace: adjust a function's pc to search for in check_stack() for arm64 AKASHI Takahiro <takahiro.akashi@linaro.org> - 2015-07-13 07:40 +0200
    Re: [RFC 1/3] ftrace: adjust a function's pc to search for in check_stack()  for arm64 AKASHI Takahiro <takahiro.akashi@linaro.org> - 2015-07-15 02:30 +0200

#1182524 — [RFC 1/3] ftrace: adjust a function's pc to search for in check_stack() for arm64

Ftace's stack tracer on arm64 returns wrong information about call stacks:

        Depth    Size   Location    (50 entries)
        -----    ----   --------
  0)     5256       0   notifier_call_chain+0x30/0x94
  1)     5256       0   ftrace_call+0x0/0x4
  2)     5256       0   notifier_call_chain+0x2c/0x94
  3)     5256       0   raw_notifier_call_chain+0x34/0x44
  4)     5256       0   timekeeping_update.constprop.9+0xb8/0x114
  5)     5256       0   update_wall_time+0x408/0x6dc

Most of 'Size' fields are unexpectedly zero.

This is because stack tracer fails to recognize each function's stack frame
in check_stack(). Stack tracer searches for a function's pc in the stack
based on the list returned by save_stack_trace(), but save_stack_trace() on
arm64 does not return the exact return address saved in a stack frame, but
a value decrmented by 4 (which means a branch instruction's address).
This behavior was introduced by
    commit e306dfd06fcb ("ARM64: unwind: Fix PC calculation")

So the matching doesn't succeed in most cases.

This problem can be fixed either by
a) reverting the commit above
b) adding an arm64-specific hack to check_patch()

This patch does b).

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
---
 kernel/trace/trace_stack.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index 3f34496..7086fc3 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -143,7 +143,11 @@ check_stack(unsigned long ip, unsigned long *stack)
 		p = start;
 
 		for (; p < top && i < max_stack_trace.nr_entries; p++) {
+#ifdef CONFIG_ARM64
+			if (*p == (stack_dump_trace[i] + 4)) {
+#else
 			if (*p == stack_dump_trace[i]) {
+#endif
 				this_size = stack_dump_index[i++] =
 					(top - p) * sizeof(unsigned long);
 				found = 1;
-- 
1.7.9.5

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [next] | [standalone]

#1184155 — Re: [RFC 1/3] ftrace: adjust a function's pc to search for in check_stack() for arm64

On 07/14/2015 12:24 AM, Steven Rostedt wrote:
> On Mon, 13 Jul 2015 14:29:33 +0900
> AKASHI Takahiro <takahiro.akashi@linaro.org> wrote:
>
>> Ftace's stack tracer on arm64 returns wrong information about call stacks:
>>
>>          Depth    Size   Location    (50 entries)
>>          -----    ----   --------
>>    0)     5256       0   notifier_call_chain+0x30/0x94
>>    1)     5256       0   ftrace_call+0x0/0x4
>>    2)     5256       0   notifier_call_chain+0x2c/0x94
>>    3)     5256       0   raw_notifier_call_chain+0x34/0x44
>>    4)     5256       0   timekeeping_update.constprop.9+0xb8/0x114
>>    5)     5256       0   update_wall_time+0x408/0x6dc
>>
>> Most of 'Size' fields are unexpectedly zero.
>>
>> This is because stack tracer fails to recognize each function's stack frame
>> in check_stack(). Stack tracer searches for a function's pc in the stack
>> based on the list returned by save_stack_trace(), but save_stack_trace() on
>> arm64 does not return the exact return address saved in a stack frame, but
>> a value decrmented by 4 (which means a branch instruction's address).
>> This behavior was introduced by
>>      commit e306dfd06fcb ("ARM64: unwind: Fix PC calculation")
>>
>> So the matching doesn't succeed in most cases.
>>
>> This problem can be fixed either by
>> a) reverting the commit above
>> b) adding an arm64-specific hack to check_patch()
>>
>> This patch does b).
>>
>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
>> ---
>>   kernel/trace/trace_stack.c |    4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
>> index 3f34496..7086fc3 100644
>> --- a/kernel/trace/trace_stack.c
>> +++ b/kernel/trace/trace_stack.c
>> @@ -143,7 +143,11 @@ check_stack(unsigned long ip, unsigned long *stack)
>>   		p = start;
>>
>>   		for (; p < top && i < max_stack_trace.nr_entries; p++) {
>> +#ifdef CONFIG_ARM64
>> +			if (*p == (stack_dump_trace[i] + 4)) {
>> +#else
>>   			if (*p == stack_dump_trace[i]) {
>> +#endif
>
> Instead of the ugly #ifdef in this code, please add a macro
> FTRACE_STACK_FRAME_OFFSET
>
> Then in include/linux/ftrace.h have:
>
> #ifndef FTRACE_STACK_FRAME_OFFSET
> # define FTRACE_STACK_FRAME_OFFSET 0
> #endif
>
> And in arch/arm64/include/asm/ftrace.h
>
> #define FTRACE_STACK_FRAME_OFFSET 4
>
> And then just do:
>
> 	if (*p == (stack_dump_trace[i] + FTRACE_STACK_FRAME_OFFSET)) {

Yes.

Thanks,
-Takahiro AKASHI

> -- Steve
>
>>   				this_size = stack_dump_index[i++] =
>>   					(top - p) * sizeof(unsigned long);
>>   				found = 1;
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web