Re: [PATCH] Perf: support to unwind userspace application stacks generated with thumb.

[Jiping Ma <Jiping.Ma2@windriver.com>]

On 04/16/2020 03:10 PM, Will Deacon wrote:
> On Thu, Apr 16, 2020 at 01:38:29PM +0800, Jiping Ma wrote:
>> Only push sp and lr in the stack for thumb mode. it will go
>> through the stack find sp and lr.
>>
>> Change this to the more detailed description of the patch
>>
>> Signed-off-by: Jiping Ma <jiping.ma2@windriver.com>
>> ---
>>   arch/arm64/kernel/perf_callchain.c | 36 +++++++++++++++++++++++++++---
>>   1 file changed, 33 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c
>> index bcafd7dcfe8b..97dde271c121 100644
>> --- a/arch/arm64/kernel/perf_callchain.c
>> +++ b/arch/arm64/kernel/perf_callchain.c
>> @@ -104,6 +104,30 @@ compat_user_backtrace(struct compat_frame_tail __user *tail,
>>   
>>   	return (struct compat_frame_tail __user *)compat_ptr(buftail.fp) - 1;
>>   }
>> +
>> +void
>> +user_backtrace_thumb(struct perf_callchain_entry_ctx *entry,
>> +		     struct pt_regs *regs)
>> +{
>> +	u32 sp;
>> +	u32 *sp_t;
>> +	/*
>> +	 * Only push sp, lr to stack.
>> +	 */
>> +	for (sp = regs->compat_sp; (sp < current->mm->start_stack) &&
>> +		(entry->nr < entry->max_stack); sp += 4) {
>> +		sp_t = (u32 *)(unsigned long)sp;
>> +		if ((*sp_t > regs->compat_sp) &&
>> +			(*sp_t < current->mm->start_stack)) {
>> +			if (*(sp_t + 1) < current->mm->end_code &&
>> +				*(sp_t + 1) > current->mm->start_code) {
>> +				perf_callchain_store(entry,  *(sp_t + 1)-1);
>> +				sp += 4;
>> +			}
>> +		}
>> +	}
>> +}
> This looks like a pile of fragile heuristics to me. Why don't you just use
> libunwind in userspace, the same way you'd have to if you compiled without
> framepointers?
Hi, Will

Could you share more details for libunwind in userspace? Following is 
our build command for test app.
bt_perf.thumb: arm-wrs-linux-gnueabi-gcc -march=armv7ve -mthumb 
-mfpu=neon -mfloat-abi=softfp -mcpu=cortex-a15 
--sysroot=sysroots/cortexa15t2-neon-wrs-linux-gnueabi -O0 -g -ggdb -Wall 
-funwind-tables -fno-omit-frame-pointer -frecord-gcc-switches 
-mapcs-frame bt_perf.c -o bt_perf.thumb

Thanks,
Jiping

>
> Will
>