linux-kernel - Re: [PATCH v1 06/22] powerpc/ftrace: Inline ftrace_modify

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Message-ID: <1bacf92b-2ad1-9ebe-f332-4ee9dd147088@csgroup.eu>
Date:   Wed, 4 May 2022 11:43:05 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Steven Rostedt <rostedt@...dmis.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v1 06/22] powerpc/ftrace: Inline ftrace_modify_code()



Le 18/04/2022 à 08:07, Naveen N. Rao a écrit :
> Christophe Leroy wrote:
>> Inlining ftrace_modify_code(), it increases a bit the
>> size of ftrace code but brings 5% improvment on ftrace
>> activation.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>>  arch/powerpc/kernel/trace/ftrace.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kernel/trace/ftrace.c 
>> b/arch/powerpc/kernel/trace/ftrace.c
>> index 41c45b9c7f39..98e82fa4980f 100644
>> --- a/arch/powerpc/kernel/trace/ftrace.c
>> +++ b/arch/powerpc/kernel/trace/ftrace.c
>> @@ -53,7 +53,7 @@ ftrace_call_replace(unsigned long ip, unsigned long 
>> addr, int link)
>>      return op;
>>  }
>>
>> -static int
>> +static inline int
>>  ftrace_modify_code(unsigned long ip, ppc_inst_t old, ppc_inst_t new)
>>  {
>>      ppc_inst_t replaced;
> 
> I thought gcc was free to inline functions without the need for 
> 'inline'. Don't you see this being inlined otherwise?

Yep, gcc is free to inline, but in that case it doesn't inline unless 
you suggest it to do so.

> 
> On the flip side, don't we need __always_inline if we want to force 
> inlining?

The question is, do we want to force inlining, even with 
CONFIG_CC_OPTIMIZE_FOR_SIZE ?

With 'inline', gcc seems to now inline it with 
CONFIG_CC_OPTIMIZE_FOR_SPEED and still keep it out of line with 
CONFIG_CC_OPTIMIZE_FOR_SIZE.

Christophe