From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrea Arcangeli <andrea@qumranet.com>
Subject: [ofa-general] Re: [PATCH 01 of 12] Core of mmu notifiers
Date: Tue, 22 Apr 2008 17:15:30 +0200
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	<480DFC8A.8040105@cosmosbay.com>
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Cc: Nick Piggin <npiggin@suse.de>, Rusty Russell <rusty@rustcorp.com.au>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>, kvm-devel@lists.sourceforge.net,
	Kanoj Sarcar <kanojsarcar@yahoo.com>, Roland Dreier <rdreier@cisco.com>,
	Jack Steiner <steiner@sgi.com>, linux-kernel@vger.kernel.org,
	Avi Kivity <avi@qumranet.com>, linux-mm@kvack.org,
	Robin Holt <holt@sgi.com>, general@lists.openfabrics.org,
	Hugh Dickins <hugh@veritas.com>, akpm@linux-foundation.org,
	Christoph Lameter <clameter@sgi.com>
To: Eric Dumazet <dada1@cosmosbay.com>
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On Tue, Apr 22, 2008 at 04:56:10PM +0200, Eric Dumazet wrote:
> Andrea Arcangeli a =E9crit :
>> +
>> +static int mm_lock_cmp(const void *a, const void *b)
>> +{
>> +	cond_resched();
>> +	if ((unsigned long)*(spinlock_t **)a <
>> +	    (unsigned long)*(spinlock_t **)b)
>> +		return -1;
>> +	else if (a =3D=3D b)
>> +		return 0;
>> +	else
>> +		return 1;
>> +}
>> +
> This compare function looks unusual...
> It should work, but sort() could be faster if the
> if (a =3D=3D b) test had a chance to be true eventually...

Hmm, are you saying my mm_lock_cmp won't return 0 if a=3D=3Db?

> static int mm_lock_cmp(const void *a, const void *b)
> {
> 	unsigned long la =3D (unsigned long)*(spinlock_t **)a;
> 	unsigned long lb =3D (unsigned long)*(spinlock_t **)b;
>
> 	cond_resched();
> 	if (la < lb)
> 		return -1;
> 	if (la > lb)
> 		return 1;
> 	return 0;
> }

If your intent is to use the assumption that there are going to be few
equal entries, you should have used likely(la > lb) to signal it's
rarely going to return zero or gcc is likely free to do whatever it
wants with the above. Overall that function is such a slow path that
this is going to be lost in the noise. My suggestion would be to defer
microoptimizations like this after 1/12 will be applied to mainline.

Thanks!