Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

Peter Zijlstra
2020-01-24T15:13:51Z

On Fri, Jan 24, 2020 at 09:42:42AM -0500, Waiman Long wrote:
> On 1/24/20 2:52 AM, Peter Zijlstra wrote:
> > On Thu, Jan 23, 2020 at 04:33:54PM -0500, Alex Kogan wrote:
> >> Let me put this question to you. What do you think the number should be?
> > I think it would be very good to keep the inter-node latency below 1ms.
> It is hard to guarantee that given that lock hold times can vary quite a
> lot depending on the workload. What we can control is just how many
> later lock waiters can jump ahead before a given waiter.

We're not into this for easy. And exactly because it depends on a lot we
need a lot of data.

Worst case lock acquisition times directly translate into worst case
IRQ-off latencies, and even the most die hard throughput oriented
workloads don't like to experience multiple ticks worth of irq-off
latencies.

> > But to realize that we need data on the lock hold times. Specifically
> > for the heavily contended locks that make CNA worth it in the first
> > place.
> >
> > I don't see that data, so I don't see how we can argue about this let
> > alone call something reasonable.
> >
> In essence, CNA lock is for improving throughput on NUMA machines at the
> expense of increasing worst case latency. If low latency is important,

Latency is _always_ important. Otherwise we'd never have put so much
time and effort into fair locks to begin with. Unbounded latency sucks
unconditionally.

> it should be disabled. If CONFIG_PREEMPT_RT is on,
> CONFIG_NUMA_AWARE_SPINLOCKS should be off.

You're spouting nonsense. You cannot claim any random number is
reasonable without argument.