Re: [PATCH 3/3] locking/qspinlock: Introduce starvation avoidance into CNA

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 30, 2019 at 10:01:35PM -0500, Alex Kogan wrote:
> Choose the next lock holder among spinning threads running on the same
> socket with high probability rather than always. With small probability,
> hand the lock to the first thread in the secondary queue or, if that
> queue is empty, to the immediate successor of the current lock holder
> in the main queue.  Thus, assuming no failures while threads hold the
> lock, every thread would be able to acquire the lock after a bounded
> number of lock transitions, with high probability.
> 
> Note that we could make the inter-socket transition deterministic,
> by sticking a counter of intra-socket transitions in the head node
> of the secondary queue. At the handoff time, we could increment
> the counter and check if it is below a threshold. This adds another
> field to queue nodes and nearly-certain local cache miss to read and
> update this counter during the handoff. While still beating stock,
> this variant adds certain overhead over the probabilistic variant.

(also heavily suffers from the socket == node confusion)

How would you suggest RT 'tunes' this?

RT relies on FIFO fairness of the basic spinlock primitives; you just
completely wrecked that.