The target to tune PCP high automatically is as follows,

- Minimize allocation/freeing from/to shared zone

- Minimize idle pages in PCP

- Minimize pages in PCP if the system free pages is too few

To reach these target, a tuning algorithm as follows is designed,

- When we refill PCP via allocating from the zone, increase PCP high.
  Because if we had larger PCP, we could avoid to allocate from the
  zone.

- In periodic vmstat updating kworker (via refresh_cpu_vm_stats()),
  decrease PCP high to try to free possible idle PCP pages.

- When page reclaiming is active for the zone, stop increasing PCP
  high in allocating path, decrease PCP high and free some pages in
  freeing path.

So, the PCP high can be tuned to the page allocating/freeing depth of
workloads eventually.

One issue of the algorithm is that if the number of pages allocated is
much more than that of pages freed on a CPU, the PCP high may become the
maximal value even if the allocating/freeing depth is small.  But this
isn't a severe issue, because there are no idle pages in this case.

One alternative choice is to increase PCP high when we drain PCP via
trying to free pages to the zone, but don't increase PCP high during PCP
refilling.  This can avoid the issue above.  But if the number of pages
allocated is much less than that of pages freed on a CPU, there will be
many idle pages in PCP and it is hard to free these idle pages.

1/8 (>> 3) of PCP high will be decreased periodically.  The value 1/8 is
kind of arbitrary.  Just to make sure that the idle PCP pages will be
freed eventually.

On a 2-socket Intel server with 224 logical CPU, we run 8 kbuild instances
in parallel (each with `make -j 28`) in 8 cgroup.  This simulates the
kbuild server that is used by 0-Day kbuild service.  With the patch, the
build time decreases 3.5%.  The cycles% of the spinlock contention (mostly
for zone lock) decreases from 11.0% to 0.5%.  The number of PCP draining
for high order pages freeing (free_high) decreases 65.6%.  The number of
pages allocated from zone (instead of from PCP) decreases 83.9%.

Link: https://lkml.kernel.org/r/20231016053002.756205-8-ying.huang@intel.comSigned-off-by: "Huang, Ying" <ying.huang@intel.com>
Suggested-by: Mel Gorman <mgorman@techsingularity.net>
Suggested-by: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>