On 02/02/2016 04:19 PM, Davidlohr Bueso wrote:
On Mon, 01 Feb 2016, Peter Zijlstra wrote:
Subject: locking/mutex: Avoid spinner vs waiter starvation
From: Peter Zijlstra <pet...@infradead.org>
Date: Fri, 22 Jan 2016 12:06:53 +0100
Ding Tianhong reported that under his load the optimistic spinners
would totally starve a task that ended up on the wait list.
Fix this by ensuring the top waiter also partakes in the optimistic
spin queue.
There are a few subtle differences between the assumed state of
regular optimistic spinners and those already on the wait list, which
result in the @acquired complication of the acquire path.
Most notable are:
- waiters are on the wait list and need to be taken off
- mutex_optimistic_spin() sets the lock->count to 0 on acquire
even though there might be more tasks on the wait list.
Right, the main impact I see with these complications are that the
window of when a waiter takes the lock via spinning and then acquires
the wait_lock to remove itself from the list, will allow an unlock
thread to set the lock as available in the fastpath which could in
turn allow a third thread the steal the lock. With high contention,
this window will be come obviously larger as we contend for the
wait_lock.
CPU-0 CPU-1 CPU-3
__mutex_lock_common mutex_optimistic_spin
(->count now 0)
__mutex_fastpath_unlock
(->count now 1) __mutex_fastpath_lock
(stolen)
spin_lock_mutex(&lock->wait_lock, flags);
But we've always been bad when it comes to counter and waiters.
Thanks,
Davidlohr
I don't quite get how that can happen. CPU0 cannot change the count to 0
unless CPU1, the lock holder, does the unlock first. Once CPU0 sees a
count of 1 and change it to 0, it is the lock holder and there can be no
other CPU that can do the unlock.
Cheers,
Longman
