linux-kernel - Re: [PATCH] arm64: perf: Fix callchain parse error with kernel tracepoint events

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Date:	Thu, 30 Apr 2015 09:50:05 +0800
From:	Hou Pengyang <houpengyang@...wei.com>
To:	Will Deacon <will.deacon@....com>
CC:	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"acme@...nel.org" <acme@...nel.org>,
	"wangnan0@...wei.com" <wangnan0@...wei.com>,
	"Catalin Marinas" <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, <mark.rutland@....com>
Subject: Re: [PATCH] arm64: perf: Fix callchain parse error with kernel tracepoint
 events

On 2015/4/29 18:12, Will Deacon wrote:
> Hello,
>
> On Tue, Apr 28, 2015 at 02:20:48PM +0100, Hou Pengyang wrote:
>> For ARM64, when tracing with tracepoint events, the IP and cpsr are set
>> to 0, preventing the perf code parsing the callchain and resolving the
>> symbols correctly.
>>
>>   ./perf record -e sched:sched_switch -g --call-graph dwarf ls
>> 	[ perf record: Captured and wrote 0.146 MB perf.data ]
>>   ./perf report -f
>>      Samples: 194  of event 'sched:sched_switch', Event count (approx.): 194
>>      Children      Self    Command  Shared Object     Symbol
>> 	100.00%       100.00%  ls       [unknown]         [.] 0000000000000000
>>
>> The fix is to implement perf_arch_fetch_caller_regs for ARM64, which fills
>> several necessary registers used for callchain unwinding, including pc,sp,
>> fp and psr .
>>
>> With this patch, callchain can be parsed correctly as follows:
>>
>>       ......
>> +    2.63%     0.00%  ls       [kernel.kallsyms]  [k] vfs_symlink
>> +    2.63%     0.00%  ls       [kernel.kallsyms]  [k] follow_down
>> +    2.63%     0.00%  ls       [kernel.kallsyms]  [k] pfkey_get
>> +    2.63%     0.00%  ls       [kernel.kallsyms]  [k] do_execveat_common.isra.33
>> -    2.63%     0.00%  ls       [kernel.kallsyms]  [k] pfkey_send_policy_notify
>>       pfkey_send_policy_notify
>>       pfkey_get
>>       v9fs_vfs_rename
>>       page_follow_link_light
>>       link_path_walk
>>       el0_svc_naked
>>      .......
>>
>> For tracepoint event, stack parsing also doesn't work well for ARM. Jean Pihet
>> comed up a patch:
>> http://thread.gmane.org/gmane.linux.kernel/1734283/focus=1734280
>
> Any chance you could revive that series too, please? I'd like to update both
> arm and arm64 together, since we're currently working at merging the two
> perf backends and introducing discrepencies is going to delay that even
> longer.
>
>> Signed-off-by: Hou Pengyang <houpengyang@...wei.com>
>> ---
>>   arch/arm64/include/asm/perf_event.h | 16 ++++++++++++++++
>>   1 file changed, 16 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/perf_event.h b/arch/arm64/include/asm/perf_event.h
>> index d26d1d5..16a074f 100644
>> --- a/arch/arm64/include/asm/perf_event.h
>> +++ b/arch/arm64/include/asm/perf_event.h
>> @@ -24,4 +24,20 @@ extern unsigned long perf_misc_flags(struct pt_regs *regs);
>>   #define perf_misc_flags(regs)	perf_misc_flags(regs)
>>   #endif
>>
>> +#define perf_arch_fetch_caller_regs(regs, __ip) { \
>> +   unsigned long sp;   \
>> +   __asm__ ("mov %[sp], sp\n" : [sp] "=r" (sp)); \
>> +       (regs)->pc = (__ip);    \
>> +       __asm__ (      \
>> +               "str %[sp],  %[_arm64_sp]  \n\t"    \
>> +               "str x29, %[_arm64_fp]  \n\t"    \
>> +               "mrs %[_arm64_cpsr], spsr_el1 \n\t"     \
>> +               : [_arm64_sp] "=m" (regs->sp),      \
>> +                 [_arm64_fp] "=m" (regs->regs[29]),  \
>> +                 [_arm64_cpsr] "=r" (regs->pstate) \
>
> Does this really all need to be in assembly code? Ideally we'd use something
> like __builtin_stack_pointer and __builtin_frame_pointer. That just leaves
> the CPSR, but given that it's (a) only used for user_mode_regs tests and (b)
> this macro is only used by ftrace, then we just set it to a static value
> indicating that we're at EL1.
>
> So I *think* we should be able to write this as three lines of C.
>
Hi, will, as you said, we can get fp by __builtin_frame_address() and 
pstate by setting it to a static value. However, for sp, there isn't a 
gcc builtin fuction like __builtin_stack_pointer, so assembly code is 
needed. What's more, if CONFIG_FRAME_POINTER is close, can fp be got by 
__builtin_frame_address()?

> Will
>
> .
>


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