linux-kernel - Re: [PATCH 5/7] powerpc/ftrace: Update ftrace

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Date:   Wed, 19 Jun 2019 13:26:37 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 5/7] powerpc/ftrace: Update ftrace_location() for powerpc
 -mprofile-kernel

Steven Rostedt wrote:
> On Tue, 18 Jun 2019 23:53:11 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
> 
>> Naveen N. Rao wrote:
>> > Steven Rostedt wrote:  
>> >> On Tue, 18 Jun 2019 20:17:04 +0530
>> >> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
>> >>   
>> >>> @@ -1551,7 +1551,7 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
>> >>>  	key.flags = end;	/* overload flags, as it is unsigned long */
>> >>>  
>> >>>  	for (pg = ftrace_pages_start; pg; pg = pg->next) {
>> >>> -		if (end < pg->records[0].ip ||
>> >>> +		if (end <= pg->records[0].ip ||  
>> >> 
>> >> This breaks the algorithm. "end" is inclusive. That is, if you look for
>> >> a single byte, where "start" and "end" are the same, and it happens to
>> >> be the first ip on the pg page, it will be skipped, and not found.  
>> > 
>> > Thanks. It looks like I should be over-riding ftrace_location() instead.  
>> > I will update this patch.  
>> 
>> I think I will have ftrace own the two instruction range, regardless of 
>> whether the preceding instruction is a 'mflr r0' or not. This simplifies 
>> things and I don't see an issue with it as of now. I will do more 
>> testing to confirm.
>> 
>> - Naveen
>> 
>> 
>> --- a/arch/powerpc/kernel/trace/ftrace.c
>> +++ b/arch/powerpc/kernel/trace/ftrace.c
>> @@ -951,6 +951,16 @@ void arch_ftrace_update_code(int command)
>>  }
>>  
>>  #ifdef CONFIG_MPROFILE_KERNEL
>> +/*
>> + * We consider two instructions -- 'mflr r0', 'bl _mcount' -- to be part
>> + * of ftrace. When checking for the first instruction, we want to include
>> + * the next instruction in the range check.
>> + */
>> +unsigned long ftrace_location(unsigned long ip)
>> +{
>> +	return ftrace_location_range(ip, ip + MCOUNT_INSN_SIZE);
>> +}
>> +
>>  /* Returns 1 if we patched in the mflr */
>>  static int __ftrace_make_call_prep(struct dyn_ftrace *rec)
>>  {
>> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
>> index 21d8e201ee80..122e2bb4a739 100644
>> --- a/kernel/trace/ftrace.c
>> +++ b/kernel/trace/ftrace.c
>> @@ -1573,7 +1573,7 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
>>   * the function tracer. It checks the ftrace internal tables to
>>   * determine if the address belongs or not.
>>   */
>> -unsigned long ftrace_location(unsigned long ip)
>> +unsigned long __weak ftrace_location(unsigned long ip)
>>  {
>>  	return ftrace_location_range(ip, ip);
>>  }
> 
> Actually, instead of making this a weak function, let's do this:
> 
> 
> In include/ftrace.h:
> 
> #ifndef FTRACE_IP_EXTENSION
> # define FTRACE_IP_EXTENSION	0
> #endif
> 
> 
> In arch/powerpc/include/asm/ftrace.h
> 
> #define FTRACE_IP_EXTENSION	MCOUNT_INSN_SIZE
> 
> 
> Then we can just have:
> 
> unsigned long ftrace_location(unsigned long ip)
> {
> 	return ftrace_location_range(ip, ip + FTRACE_IP_EXTENSION);
> }

Thanks, that's indeed nice. I hope you don't mind me adding your SOB for 
that.

- Naveen