> 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, — Alex
