contention on pwq->pool->lock under heavy NFS workload

[Chuck Lever III]

Hi Tejun-

lock_stat reports that the pool->lock kernel/workqueue.c:1483 is the highest
contended lock on my test NFS client. The issue appears to be that the three
NFS-related workqueues, rpciod_workqueue, xprtiod_workqueue, and nfsiod all
get placed in the same worker_pool, so they have to fight over one pool lock.

I notice that ib_comp_wq is allocated with the same flags, but I don't see
significant contention there, and a trace_printk in __queue_work shows that
work items queued on that WQ seem to alternate between at least two different
worker_pools.

Is there a preferred way to ensure the NFS WQs get spread a little more fairly
amongst the worker_pools?

Thanks in advance.

--
Chuck Lever