[RFC PATCH] powerpc/64/ftrace: mprofile-kernel patch out mflr

[Michael Ellerman]

"Naveen N. Rao" <naveen.n.rao at linux.ibm.com> writes:
> Michael Ellerman wrote:
>> Nicholas Piggin <npiggin at gmail.com> writes:
>>> The new mprofile-kernel mcount sequence is
>>>
>>>   mflr	r0
>>>   bl	_mcount
>>>
>>> Dynamic ftrace patches the branch instruction with a noop, but leaves
>>> the mflr. mflr is executed by the branch unit that can only execute one
>>> per cycle on POWER9 and shared with branches, so it would be nice to
>>> avoid it where possible.
>>>
>>> This patch is a hacky proof of concept to nop out the mflr. Can we do
>>> this or are there races or other issues with it?
>> 
>> There's a race, isn't there?
>> 
>> We have a function foo which currently has tracing disabled, so the mflr
>> and bl are nop'ed out.
>> 
>>   CPU 0			CPU 1
>>   ==================================
>>   bl foo
>>   nop (ie. not mflr)
>>   -> interrupt
>>   something else	enable tracing for foo
>>   ...			patch mflr and branch
>>   <- rfi
>>   bl _mcount
>> 
>> So we end up in _mcount() but with r0 not populated.
>
> Good catch! Looks like we need to patch the mflr with a "b +8" similar 
> to what we do in __ftrace_make_nop().

Would that actually make it any faster though? Nick?

cheers