percpu: use reclaim threshold instead of running for every page

The last patch implements reclaim by adding 2 additional lists where a
chunk's lifecycle is:
  active_slot -> to_depopulate_slot -> sidelined_slot

This worked great because we're able to nicely converge paths into
isolation. However, it's a bit aggressive to run for every free page.
Let's accumulate a few free pages before we do this. To do this, the new
lifecycle is:
  active_slot -> sidelined_slot -> to_depopulate_slot -> sidelined_slot

The transition from sidelined_slot -> to_depopulate_slot occurs on a
threshold instead of before where it directly went to the
to_depopulate_slot. pcpu_nr_isolated_empty_pop_pages[] is introduced to
aid with this.

Suggested-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
3 files changed