Re: [PATCH RFC 6/8] SUNRPC: Reduce rpciod workqueue contention

[Tim Menninger]

I realized the bad state correlates with where the RDMA CQs' completion
work runs.

This client has 16 RDMA transports and 32 CQs. The CQs receive 32
consecutive completion vectors out of the mlx5 device's 63-vector ring.
Depending on the global round-robin starting point, all 32 CQ workers can
execute on NUMA node 0, or they can split 17/15 between the two NUMA nodes.

On unpatched mainline (940de590b839 without your patch set), I tested five
consecutive allocation windows. On three of those, all 32 CQs executed on
NUMA node 0. On the other two they fell with a 17/15 split. All five
sustained ~46 GB/s.

With your v2 applied, all-local windows fall to roughly 24-28 GB/s while
split windows sustain ~46 GB/s.

I also tested this by changing only mlx5 IRQ affinity. I redirected
completion vectors 20-34 from node 0 to CPUs 24-38 on node 1. CQ allocation
windows 5-36 and 6-37, which would otherwise be entirely local and slow,
then sustained ~46 GB/s with their CQs split 17/15 across the nodes.

So the CQ allocation determines whether the regression is exposed, but
mainline is insensitive to that placement. The series introduces the
performance sensitivity. This also explains why v2 appeared
nondeterministic with respect to the rpciod affinity scope alone.