linux-kernel - Re: [PATCH] fs: inode per-cpu last

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Date:	Thu, 30 Sep 2010 20:05:10 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: inode per-cpu last_ino allocator

Le jeudi 30 septembre 2010 à 10:39 -0700, Andrew Morton a écrit :
> On Thu, 30 Sep 2010 19:28:05 +0200 Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > Le jeudi 30 septembre 2010 __ 09:45 -0700, Andrew Morton a __crit :
> > 
> > > Could eliminate `p' I guess, but that would involve using
> > > __get_cpu_var() as an lval, which looks vile and might generate worse
> > > code.
> > > 
> > 
> > Hmm, I see, please check this new patch, using the most modern stuff ;)
> > 
> > > Readers of this code won't know why last_ino_get() was marked noinline.
> > > It looks wrong, really.
> > 
> > Oops sorry, this was a temporary hack of mine to ease disassembly
> > analysis. Good catch !
> > 
> > Here is the new generated code on i686 (with the noinline) : 
> > pretty good ;)
> > 
> > c02e5930 <last_ino_get>:
> > c02e5930:	55                   	push   %ebp
> > c02e5931:	89 e5                	mov    %esp,%ebp
> > c02e5933:	64 a1 44 29 7d c0    	mov    %fs:0xc07d2944,%eax
> > c02e5939:	a9 ff 03 00 00       	test   $0x3ff,%eax
> > c02e593e:	74 09                	je     c02e5949 <last_ino_get+0x19>
> > c02e5940:	40                   	inc    %eax
> > c02e5941:	64 a3 44 29 7d c0    	mov    %eax,%fs:0xc07d2944
> > c02e5947:	c9                   	leave  
> > c02e5948:	c3                   	ret    
> > c02e5949:	b8 00 04 00 00       	mov    $0x400,%eax
> > c02e594e:	f0 0f c1 05 80 c8 92 c0	lock xadd %eax,0xc092c880
> > c02e5956:	eb e8                	jmp    c02e5940 <last_ino_get+0x10>
> > 
> 
> That uniprocessor, PREEMPT=n I guess.
> 

It is SMP. PREEMPT=n

On UP, you would not have the %fs suffixes, and no "lock" before the
xadd.



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