From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: David Laight <David.Laight@ACULAB.COM>,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@redhat.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net v3] net: Force inlining of checksum functions in net/checksum.h
Date: Thu, 17 Feb 2022 15:55:21 +0100 [thread overview]
Message-ID: <9b8ef186-c7fe-822c-35df-342c9e86cc88@csgroup.eu> (raw)
In-Reply-To: <d38e5e1c-29b6-8cc6-7409-d0bdd5772f23@csgroup.eu>
Le 17/02/2022 à 15:50, Christophe Leroy a écrit :
> Adding Ingo, Andrew and Nick as they were involved in the subjet,
>
> Le 17/02/2022 à 14:36, David Laight a écrit :
>> From: Christophe Leroy
>>> Sent: 17 February 2022 12:19
>>>
>>> All functions defined as static inline in net/checksum.h are
>>> meant to be inlined for performance reason.
>>>
>>> But since commit ac7c3e4ff401 ("compiler: enable
>>> CONFIG_OPTIMIZE_INLINING forcibly") the compiler is allowed to
>>> uninline functions when it wants.
>>>
>>> Fair enough in the general case, but for tiny performance critical
>>> checksum helpers that's counter-productive.
>>
>> There isn't a real justification for allowing the compiler
>> to 'not inline' functions in that commit.
>
> Do you mean that the two following commits should be reverted:
>
> - 889b3c1245de ("compiler: remove CONFIG_OPTIMIZE_INLINING entirely")
> - 4c4e276f6491 ("net: Force inlining of checksum functions in
> net/checksum.h")
Of course not the above one (copy/paste error), but:
- ac7c3e4ff401 ("compiler: enable CONFIG_OPTIMIZE_INLINING forcibly")
>
>>
>> It rather seems backwards.
>> The kernel sources don't really have anything marked 'inline'
>> that shouldn't always be inlined.
>> If there are any such functions they are few and far between.
>>
>> I've had enough trouble (elsewhere) getting gcc to inline
>> static functions that are only called once.
>> I ended up using 'always_inline'.
>> (That is 4k of embedded object code that will be too slow
>> if it ever spills a register to stack.)
>>
>
> I agree with you that that change is a nightmare with many small
> functions that we really want inlined, and when we force inlining we
> most of the time get a smaller binary.
>
> And it becomes even more problematic when we start adding
> instrumentation like stack protector.
>
> According to the original commits however this was supposed to provide
> real benefit:
>
> - 60a3cdd06394 ("x86: add optimized inlining")
> - 9012d011660e ("compiler: allow all arches to enable
> CONFIG_OPTIMIZE_INLINING")
>
> But when I build ppc64le_defconfig + CONFIG_CC_OPTIMISE_FOR_SIZE I get:
> 112 times queued_spin_unlock()
> 122 times mmiowb_spin_unlock()
> 151 times cpu_online()
> 225 times __raw_spin_unlock()
>
>
> So I was wondering, would we have a way to force inlining of functions
> marked inline in header files while leaving GCC handling the ones in C
> files the way it wants ?
>
> Christophe
next prev parent reply other threads:[~2022-02-17 14:55 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-17 12:19 [PATCH net v3] net: Force inlining of checksum functions in net/checksum.h Christophe Leroy
2022-02-17 13:36 ` David Laight
2022-02-17 14:50 ` Christophe Leroy
2022-02-17 14:55 ` Christophe Leroy [this message]
2022-02-17 15:15 ` David Laight
2022-02-17 16:17 ` Masahiro Yamada
2022-02-17 16:49 ` David Laight
2022-02-17 17:27 ` Masahiro Yamada
2022-02-17 18:07 ` Segher Boessenkool
2022-02-18 1:35 ` Masahiro Yamada
2022-02-18 12:12 ` Segher Boessenkool
2022-02-18 16:29 ` Stephen Hemminger
2022-02-18 16:44 ` Segher Boessenkool
2022-02-18 8:41 ` David Laight
2022-02-17 15:42 ` Joe Perches
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