Re: AIM7 40% regression with 2.6.26-rc1

[Matthew Wilcox]

On Tue, May 06, 2008 at 10:21:53AM -0700, Andrew Morton wrote:
> up() seems to be doing wake-one, FIFO which is nice.  Did the
> implementation which we just removed also do that?  Was it perhaps
> accidentally doing LIFO or something like that?

If heavily contended, it could do this.

up() would increment sem->count and cal __up() which would call wake_up()
down() would decrement sem->count.
The unlucky task woken by __up would lose the race and go back to sleep.

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