vfs: distribute freevnodes counter per-cpu

[mjg]

Summary

It gets rolled up to the global when deferred requeueing is performed. A dedicated read routine makes sure to return a value only off by a certain amount.

This soothes a global serialisation point for all 0<->1 hold count transitions.

Test Plan

In my tests with kern.maxvnodes=5000 vnode reclamation stats were almost identical. On a bigger scale workload this gives the following after almost 9h:

vfs.numvnodes: 5209396
vfs.freevnodes: 4790184
vfs.recycles: 93
vfs.recycles_free: 1019102
vfs.alloc_sleeps: 0
vfs.freevnode_fetches: 1800

that is, the per-cpu walk was almost never done despite reclamations being present

[mjg]

sys/kern/vfs_subr.c

3238 ↗

(On Diff #66913)

In principle this is wrong since a sufficiently nasty compiler could split this into a load, decrement and write back. then getting preempted by a thread which ends up doing vdbatch_process will end up in miscalculation -- whatever alteration was made is now lost. I don't know if we care. If we do, the simplest thing to do is critical_enter/exit dance.

[mjg]

• don't depend on the compiler, just critical enter/exit for safety

[jeff]

sys/kern/vfs_subr.c

1305 ↗

(On Diff #66934)

Please use a define for magic numbers.

[mjg]

• move slop to a macro
• add a comment explaining the read func