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Date: Fri, 17 May 2019 19:12:45 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [RFC PATCH] powerpc/64/ftrace: mprofile-kernel patch out mflr
To: linuxppc-dev@lists.ozlabs.org, Michael Ellerman <mpe@ellerman.id.au>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
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Naveen N. Rao's on May 17, 2019 4:22 am:
> Nicholas Piggin wrote:
>> Naveen N. Rao's on May 14, 2019 6:32 pm:
>>> Michael Ellerman wrote:
>>>> "Naveen N. Rao" <naveen.n.rao@linux.ibm.com> writes:
>>>>> Michael Ellerman wrote:
>>>>>> Nicholas Piggin <npiggin@gmail.com> writes:
>>>>>>> The new mprofile-kernel mcount sequence is
>>>>>>>
>>>>>>>   mflr	r0
>>>>>>>   bl	_mcount
>>>>>>>
>>>>>>> Dynamic ftrace patches the branch instruction with a noop, but leav=
es
>>>>>>> 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 t=
o
>>>>>>> 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?
>>>>>>=20
>>>>>> There's a race, isn't there?
>>>>>>=20
>>>>>> We have a function foo which currently has tracing disabled, so the =
mflr
>>>>>> and bl are nop'ed out.
>>>>>>=20
>>>>>>   CPU 0			CPU 1
>>>>>>   =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>>>>>>   bl foo
>>>>>>   nop (ie. not mflr)
>>>>>>   -> interrupt
>>>>>>   something else	enable tracing for foo
>>>>>>   ...			patch mflr and branch
>>>>>>   <- rfi
>>>>>>   bl _mcount
>>>>>>=20
>>>>>> 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" simila=
r=20
>>>>> to what we do in __ftrace_make_nop().
>>>>=20
>>>> Would that actually make it any faster though? Nick?
>>>=20
>>> Ok, how about doing this as a 2-step process?
>>> 1. patch 'mflr r0' with a 'b +8'
>>>    synchronize_rcu_tasks()
>>> 2. convert 'b +8' to a 'nop'
>>=20
>> Good idea. Well the mflr r0 is harmless, so you can leave that in.
>> You just need to ensure it's not removed before the bl is. So nop
>> the bl _mcount, then synchronize_rcu_tasks(), then nop the mflr?
>=20
> The problem actually seems to be when we try to patch in the branch to=20
> _mcount(), rather than when we are patching in the nop instructions=20
> (i.e., the race is when we try to enable the function tracer, rather=20
> than while disabling it).
>=20
> When we disable ftrace, we only need to ensure we patch out the branch=20
> to _mcount() before patching out the preceding 'mflr r0'. I don't think=20
> we need a synchronize_rcu_tasks() in that case.

That's probably right.

> While enabling ftrace, we will first need to patch the preceding 'mflr=20
> r0' (which would now be a 'nop') with 'b +8', then use=20
> synchronize_rcu_tasks() and finally patch in 'bl _mcount()' followed by=20
> 'mflr r0'.
>=20
> I think that's what you meant, just that my reference to patching 'mflr=20
> r0' with a 'b +8' should have called out that the mflr would have been=20
> nop'ed out.

I meant that we don't need the b +8 anywhere, because the mflr r0
is harmless. Enabling ftrace just needs to patch in 'mflr r0', and
then synchronize_rcu_tasks(), and then patch in 'bl _mcount'. I think?

Thanks,
Nick

=