[External] Re: [PATCH] arm64/ftrace: Make function graph use ftrace directly

[Chengming Zhou]

On 2022/2/22 11:54 下午, Mark Rutland wrote:
> On Tue, Feb 22, 2022 at 09:00:49PM +0800, Chengming Zhou wrote:
>> As we do in commit 0c0593b45c9b ("x86/ftrace: Make function graph
>> use ftrace directly"), we don't need special hook for graph tracer,
>> but instead we use graph_ops:func function to install return_hooker.
>>
>> Since commit 3b23e4991fb6 ("arm64: implement ftrace with regs") add
>> implementation for FTRACE_WITH_REGS on arm64, we can easily adopt
>> the same optimization on arm64.
> 
> This is a nice cleanup/refactoring, but I don't think this is an
> optimization as such; we're still doing the same work, just in
> marginally different place. So I'd suggest s/optimization/cleanup/ here.
> 
> It's probably worth noting that this *only* changes the FTRACE_WITH_REGS
> implementation, and the mcount-based implementation is unaffected by
> this patch.
> 

Agree, I will change the commit message in the next version.

>> Signed-off-by: Chengming Zhou <zhouchengming at bytedance.com>
>> ---
>>  arch/arm64/include/asm/ftrace.h  |  7 +++++++
>>  arch/arm64/kernel/entry-ftrace.S |  6 ------
>>  arch/arm64/kernel/ftrace.c       | 21 +++++++++++++++++++++
>>  3 files changed, 28 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
>> index 1494cfa8639b..dbc45a4157fa 100644
>> --- a/arch/arm64/include/asm/ftrace.h
>> +++ b/arch/arm64/include/asm/ftrace.h
>> @@ -80,8 +80,15 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
>>  
>>  #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>>  struct dyn_ftrace;
>> +struct ftrace_ops;
>> +struct ftrace_regs;
>> +
>>  int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
>>  #define ftrace_init_nop ftrace_init_nop
>> +
>> +void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
>> +		       struct ftrace_ops *op, struct ftrace_regs *fregs);
>> +#define ftrace_graph_func ftrace_graph_func
>>  #endif
>>  
>>  #define ftrace_return_address(n) return_address(n)
>> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
>> index e535480a4069..eb4a69b1f84d 100644
>> --- a/arch/arm64/kernel/entry-ftrace.S
>> +++ b/arch/arm64/kernel/entry-ftrace.S
>> @@ -97,12 +97,6 @@ SYM_CODE_START(ftrace_common)
>>  SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
>>  	bl	ftrace_stub
>>  
>> -#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> -SYM_INNER_LABEL(ftrace_graph_call, SYM_L_GLOBAL) // ftrace_graph_caller();
>> -	nop				// If enabled, this will be replaced
>> -					// "b ftrace_graph_caller"
>> -#endif
>> -
> 
> You should also be able to delete the FTRACE_WITH_REGS implementation of
> ftrace_graph_caller since that's now unused.
> 
> Having that in the diff would also make it easier to compare to the
> logic in ftrace_graph_func().
> 

Yes, will do.

>>  /*
>>   * At the callsite x0-x8 and x19-x30 were live. Any C code will have preserved
>>   * x19-x29 per the AAPCS, and we created frame records upon entry, so we need
>> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
>> index 4506c4a90ac1..1b5da231b1de 100644
>> --- a/arch/arm64/kernel/ftrace.c
>> +++ b/arch/arm64/kernel/ftrace.c
>> @@ -268,6 +268,26 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
>>  }
>>  
>>  #ifdef CONFIG_DYNAMIC_FTRACE
>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>> +int ftrace_enable_ftrace_graph_caller(void)
>> +{
>> +	return 0;
>> +}
>> +
>> +int ftrace_disable_ftrace_graph_caller(void)
>> +{
>> +	return 0;
>> +}
> 
> It's a shame the core code doesn't provide this if we provide an
> implementation of ftrace_graph_func.
> 

Maybe I can provide these weak version functions in the ftrace core code
with an additional patch.

>> +
>> +void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
>> +		       struct ftrace_ops *op, struct ftrace_regs *fregs)
>> +{
>> +	struct pt_regs *regs = arch_ftrace_get_regs(fregs);
>> +	unsigned long *parent = (unsigned long *)&procedure_link_pointer(regs);
>> +
>> +	prepare_ftrace_return(ip, parent, frame_pointer(regs));
>> +}
> 
> Other than my comments above, this looks about right, but I'd like to
> give this some testing before I give any tags.
> 
> Could you respin this with the FTRACE_WITH_REGS ftrace_graph_caller asm
> removed?
> 
> Thanks,
> Mark.
> 
Of course, will do.

Thanks!

>> +#else
>>  /*
>>   * Turn on/off the call to ftrace_graph_caller() in ftrace_caller()
>>   * depending on @enable.
>> @@ -297,5 +317,6 @@ int ftrace_disable_ftrace_graph_caller(void)
>>  {
>>  	return ftrace_modify_graph_caller(false);
>>  }
>> +#endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
>>  #endif /* CONFIG_DYNAMIC_FTRACE */
>>  #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
>> -- 
>> 2.20.1
>>