Andrea Arcangeli a écrit :
On Tue, Apr 22, 2008 at 04:56:10PM +0200, Eric Dumazet wrote:
Andrea Arcangeli a écrit :
+
+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 == b)
+ return 0;
+ else
+ return 1;
+}
+
This compare function looks unusual...
It should work, but sort() could be faster if the
if (a == b) test had a chance to be true eventually...
Hmm, are you saying my mm_lock_cmp won't return 0 if a==b?
I am saying your intent was probably to test
else if ((unsigned long)*(spinlock_t **)a ==
(unsigned long)*(spinlock_t **)b)
return 0;
Because a and b are pointers to the data you want to compare. You need
to dereference them.
static int mm_lock_cmp(const void *a, const void *b)
{
unsigned long la = (unsigned long)*(spinlock_t **)a;
unsigned long lb = (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!
Hum, it's not a micro-optimization, but a bug fix. :)
Sorry if it was not clear
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