uma: Use a taskqueue to execute uma_timeout()

[markj]

Summary

uma_timeout() has several responsibilities; it visits every UMA zone and
as of recently will drain underutilized caches, so is rather expensive
(>1ms in some cases). Currently it is executed by softclock threads
and so will preempt most other CPU activity. None of this work requires
a high scheduling priority, though, so defer it to a taskqueue so as to
avoid stalling higher-priority work.

Sponsored by: The FreeBSD Foundation

[kib]

It might be reasonable to create a dedicated taskqueue thread for UMA, sometime later.

[markj]

In D35738#810793, @kib wrote:

It might be reasonable to create a dedicated taskqueue thread for UMA, sometime later.

Yes, I think so. We already have a dedicated UMA thread which executes uma_reclaim_worker().

[rlibby]

sys/vm/uma_core.c

1110

Since both arguments are unused, maybe void *context __unused reads better for the first argument?

[markj]

Use better parameter names.