[External] Re: [PATCH v4 1/2] ftrace: cleanup ftrace_graph_caller enable and disable

[Chengming Zhou]

Hi,

On 2022/4/19 19:41, Mark Rutland wrote:
> On Sat, Apr 09, 2022 at 11:35:53PM +0800, Chengming Zhou wrote:
>> The ftrace_[enable,disable]_ftrace_graph_caller() are used to do
>> special hooks for graph tracer, which are not needed on some ARCHs
>> that use graph_ops:func function to install return_hooker.
>>
>> So introduce the weak version in ftrace core code to cleanup
>> in x86.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming at bytedance.com>
>> ---
>> v4:
>>  - put weak ftrace_enable,disable_ftrace_graph_caller() in
>>    fgraph.c instead of ftrace.c as suggested by Steve.
>>
>> v3:
>>  - consolidate two #if into a single #if, suggested by Steve. Thanks.
>> ---
>>  arch/x86/kernel/ftrace.c | 17 ++---------------
>>  kernel/trace/fgraph.c    | 18 ++++++++++++++++++
>>  2 files changed, 20 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
>> index 1e31c7d21597..b09d73c2ba89 100644
>> --- a/arch/x86/kernel/ftrace.c
>> +++ b/arch/x86/kernel/ftrace.c
>> @@ -579,9 +579,7 @@ void arch_ftrace_trampoline_free(struct ftrace_ops *ops)
>>  
>>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>>  
>> -#ifdef CONFIG_DYNAMIC_FTRACE
>> -
>> -#ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
>> +#if defined(CONFIG_DYNAMIC_FTRACE) && !defined(CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS)
>>  extern void ftrace_graph_call(void);
>>  static const char *ftrace_jmp_replace(unsigned long ip, unsigned long addr)
>>  {
>> @@ -610,18 +608,7 @@ int ftrace_disable_ftrace_graph_caller(void)
>>  
>>  	return ftrace_mod_jmp(ip, &ftrace_stub);
>>  }
>> -#else /* !CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS */
>> -int ftrace_enable_ftrace_graph_caller(void)
>> -{
>> -	return 0;
>> -}
>> -
>> -int ftrace_disable_ftrace_graph_caller(void)
>> -{
>> -	return 0;
>> -}
>> -#endif /* CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS */
>> -#endif /* !CONFIG_DYNAMIC_FTRACE */
>> +#endif /* CONFIG_DYNAMIC_FTRACE && !CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS */
>>  
>>  /*
>>   * Hook the return address and push it in the stack of return addrs
>> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
>> index 8f4fb328133a..289311680c29 100644
>> --- a/kernel/trace/fgraph.c
>> +++ b/kernel/trace/fgraph.c
>> @@ -30,6 +30,24 @@ int ftrace_graph_active;
>>  /* Both enabled by default (can be cleared by function_graph tracer flags */
>>  static bool fgraph_sleep_time = true;
>>  
>> +/*
>> + * archs can override this function if they must do something
>> + * to enable hook for graph tracer.
>> + */
>> +int __weak ftrace_enable_ftrace_graph_caller(void)
>> +{
>> +	return 0;
>> +}
>> +
>> +/*
>> + * archs can override this function if they must do something
>> + * to disable hook for graph tracer.
>> + */
>> +int __weak ftrace_disable_ftrace_graph_caller(void)
>> +{
>> +	return 0;
>> +}
> 
> IIUC an arch should either:
> 
> * Have ftrace_graph_call()
> 
> * Have both ftrace_enable_ftrace_graph_caller() and
>   ftrace_disable_ftrace_graph_caller()
> 
> ... and I can't think of a reason an arch would need both ftrace_graph_call()
> *and* the enable/disable functions.

This way is more precise, but we have to add "#define ftrace_graph_call ftrace_graph_call"
in all ARCH's asm/ftrace.h correctly.

Previously we only need to define override ftrace_[enable,disable]_ftrace_graph_caller()
in ARCH's ftrace.c, looks like a little more complexity?

> 
> Given that, could we drop the `__weak` and place these within ifdeffery, i.e.
> make the above:
> 
> | #ifndef ftrace_graph_call
> | int ftrace_enable_ftrace_graph_caller(void) { return 0; }
> | int ftrace_disable_ftrace_graph_caller(void) { return 0; }
> | #endif /* ftrace_graph_call *. 
> 
> That way we'd catch cases when:
> 
> * An architecture meant to provide one of these functions, but forgot (e.g. the
>   name got typo'd)
> 
> * An architecture provides an unnecessary implementation of either of these
>   functions.

I tried this way today, and it works too. I'm ok with both way. Maybe I should
send v5 with this patch unchanged first. Looking forward to Steve's opinion.

Thanks.

> 
> Regardless, this looks ok to me. Steve, are you happy with this? I suspect we'd
> need to take this via the arm64 tree with the next patch, so we'd need your Ack.
> 
> Thanks,
> Mark.
> 
>> +
>>  /**
>>   * ftrace_graph_stop - set to permanently disable function graph tracing
>>   *
>> -- 
>> 2.35.1
>>