linux-kernel - Re: spinlocks -- why are releases inlined and acquires are not?

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Message-ID: <Pine.LNX.4.64.0804011034410.5541@jikos.suse.cz>
Date:	Tue, 1 Apr 2008 10:38:41 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Ingo Molnar <mingo@...e.hu>
cc:	linux-kernel@...r.kernel.org
Subject: Re: spinlocks -- why are releases inlined and acquires are not?

On Tue, 1 Apr 2008, Ingo Molnar wrote:

> > > What is the reason for this asymetry? Shouldn't the acquiring 
> > > functions be implemented in the very same way? Or at least, 
> > > shouldn't all the __lockfunc functions be inlined?
> > i.e. is there any particular reason why we don't have something like 
> > the patch below (implemented for all the lock variants of course, this 
> > is just to demonstrate what I mean)?
> IIRC the main reason we decided to uninline them was image size. So i'd 
> suggest for you to check how this change impacts vmlinux size (on both 
> 64-bit and 32-bit), a typical distro config (or allyesconfig with lock 
> debugging disabled). If you do the test on x86.git/latest you'll also 
> have the CONFIG_OPTIMIZE_INLINING=y and CONFIG_CC_OPTIMIZE_FOR_SIZE=y 
> combination as well, which generates the most compact x86 kernel image 
> ever.

In fact we have received report from one of our users that he is seeing 
approximately 15% performance degradation of mmap() when spinlocks are not 
inlined. I am going to do some performance measurements myself shortly, as 
it seems quite strange, but while at it, I have noticed the aforementioned 
asymetry in spinlock.h, so I just wanted to know if there is any 
particular reason behind that.

-- 
Jiri Kosina
SUSE Labs
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