netdev - Re: [PATCH v2] jhash: Deinline jhash, jhash2 and __jhash

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Date:	Fri, 17 Jul 2015 15:44:45 +0200 (CEST)
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Joe Perches <joe@...ches.com>, David Miller <davem@...emloft.net>
Cc:	tom <tom@...bertland.com>, herbert <herbert@...dor.apana.org.au>,
	tgraf <tgraf@...g.ch>, netdev <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	dvlasenk <dvlasenk@...hat.com>,
	"alexander.h.duyck" <alexander.h.duyck@...hat.com>,
	kadlec <kadlec@...ckhole.kfki.hu>
Subject: Re: [PATCH v2] jhash: Deinline jhash, jhash2 and __jhash_nwords

> On July 16, 2015 at 9:23 PM Joe Perches <joe@...ches.com> wrote:
> 
> It might be useful to have these performance impacting
> changes guarded by something like CONFIG_CC_OPTIMIZE_FOR_SIZE
> with another static __always_inline __<func> and a function &
> EXPORT_SYMBOL or just a static inline so that where code size
> is critical it's uninlined.

But keep in mind that jhash, jhash2 and __jhash_nwords are *not*
one-instruction long functions. We duplicate code over and over resulting
probably in more cache misses. __always_inline__ is probably too strict
and a vanilla inline is already for 99% of all distribution builds a
 __always_inline__, see ARCH_SUPPORTS_OPTIMIZED_INLINING and
CONFIG_CC_OPTIMIZE_FOR_SIZE.

The answer depends on the specific workload. Sometimes an enforced inline
perform better and sometimes a call is the better solution (read: less
cache misses). General purpose vendors with a larger working set size
should reduce cache misses by deinline many functions. For
high-performance special fast-path operations a strong inlined kernel
build is probably faster. __always_inline__ makes it impossible for the
user to deinline functions or not.

Hagen
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