linux-kernel - Re: [RFC PATCH 6/9] powerpc/ftrace: Update and move function profile instructions out-of-line

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Date: Thu, 21 Dec 2023 15:20:14 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Steven Rostedt <rostedt@...dmis.org>
CC: Naveen N Rao <naveen@...nel.org>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>, Aneesh Kumar K.V
	<aneesh.kumar@...nel.org>, Mark Rutland <mark.rutland@....com>, Florent
 Revest <revest@...omium.org>, Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC PATCH 6/9] powerpc/ftrace: Update and move function profile
 instructions out-of-line



Le 21/12/2023 à 15:25, Steven Rostedt a écrit :
> On Thu, 21 Dec 2023 10:46:08 +0000
> Christophe Leroy <christophe.leroy@...roup.eu> wrote:
> 
>>> To enable ftrace, the nop at function entry is changed to an
>>> unconditional branch to 'tramp'. The call to ftrace_caller() may be
>>> updated to ftrace_regs_caller() depending on the registered ftrace ops.
>>> On 64-bit powerpc, we additionally change the instruction at 'tramp' to
>>> 'mflr r0' from an unconditional branch back to func+4. This is so that
>>> functions entered through the GEP can skip the function profile sequence
>>> unless ftrace is enabled.
>>>
>>> With the context_switch microbenchmark on a P9 machine, there is a
>>> performance improvement of ~6% with this patch applied, going from 650k
>>> context switches to 690k context switches without ftrace enabled. With
>>> ftrace enabled, the performance was similar at 86k context switches.
>>
>> Wondering how significant that context_switch micorbenchmark is.
>>
>> I ran it on both mpc885 and mpc8321 and I'm a bit puzzled by some of the
>> results:
>> # ./context_switch --no-fp
>> Using threads with yield on cpus 0/0 touching FP:no altivec:no vector:no
>> vdso:no
>>
>> On 885, I get the following results before and after your patch.
>>
>> CONFIG_FTRACE not selected : 44,9k
>> CONFIG_FTRACE selected, before : 32,8k
>> CONFIG_FTRACE selected, after : 33,6k
>>
>> All this is with CONFIG_INIT_STACK_ALL_ZERO which is the default. But
>> when I select CONFIG_INIT_STACK_NONE, the CONFIG_FTRACE not selected
>> result is only 34,4.
>>
>> On 8321:
>>
>> CONFIG_FTRACE not selected : 100,3k
>> CONFIG_FTRACE selected, before : 72,5k
>> CONFIG_FTRACE selected, after : 116k
>>
>> So the results look odd to me.
> 
> 
> BTW, CONFIG_FTRACE just enables the tracing system (I would like to change
> that to CONFIG_TRACING, but not sure if I can without breaking .configs all
> over the place).
> 
> The nops for ftrace is enabled with CONFIG_FUNCTION_TRACER.

Yes I selected both CONFIG_FTRACE and CONFIG_FUNCTION_TRACER

Christophe