From: Nicholas Piggin <npiggin@gmail.com>
To: linuxppc-dev@lists.ozlabs.org,
Michael Ellerman <mpe@ellerman.id.au>,
"Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Subject: Re: [RFC PATCH] powerpc/64/ftrace: mprofile-kernel patch out mflr
Date: Thu, 16 May 2019 15:49:51 +1000 [thread overview]
Message-ID: <1557985279.4o349j5g2i.astroid@bobo.none> (raw)
In-Reply-To: <1557821918.xbleq18bk2.naveen@linux.ibm.com>
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 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?
>
> 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'
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?
Thanks,
Nick
next prev parent reply other threads:[~2019-05-16 5:51 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-13 1:59 [RFC PATCH] powerpc/64/ftrace: mprofile-kernel patch out mflr Nicholas Piggin
2019-04-15 9:42 ` Naveen N. Rao
2019-05-13 3:25 ` Michael Ellerman
2019-05-13 6:47 ` Naveen N. Rao
2019-05-13 11:34 ` Michael Ellerman
2019-05-14 8:32 ` Naveen N. Rao
2019-05-15 12:20 ` Michael Ellerman
2019-05-16 5:49 ` Nicholas Piggin [this message]
2019-05-16 18:22 ` Naveen N. Rao
2019-05-17 9:12 ` Nicholas Piggin
2019-05-17 17:25 ` Naveen N. Rao
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