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

[Alex Kogan]

> On Jan 31, 2019, at 5:00 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> 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.

This is true that CNA trades some fairness for shorter lock handover latency, much like any other NUMA-aware lock.

Can you explain, however, what exactly breaks here?
It seems that even today, qspinlock does not support RT_PREEMPT, given that it uses per-CPU queue nodes.

Thank you,
â Alex