On 01/22/2016 03:54 AM, Peter Zijlstra wrote:
On Thu, Jan 21, 2016 at 06:02:34PM -0500, Waiman Long wrote:
This patch attempts to fix this live-lock condition by enabling the
a woken task in the wait list to enter optimistic spinning loop itself
with precedence over the ones in the OSQ. This should prevent the
live-lock
condition from happening.
So I think having the top waiter going back in to contend on the OSQ is
an excellent idea, but I'm not sure the wlh_spinning thing is important.
Yes, that is optional. I put it there just to make it is more likely for
the waiter spinner to get the lock. Without that, the chance will be
50/50 on average. I can certainly take that out.
The OSQ itself is FIFO fair, and the waiters retain the wait_list
position. So having the top wait_list entry contending on the OSQ
ensures we cannot starve (I think).
Also, as Davidlohr said, we cannot copy/paste this much code.
As I said in the previous mail, I do intend to refactor it before
sending out the official patch.
Cheers,
Longman