When compaction was implemented it was known that scanning could potentially
be excessive. The ideal was that a counter be maintained for each pageblock
but maintaining this information would incur a severe penalty due to a
shared writable cache line. It has reached the point where the scanning
costs are an serious problem, particularly on long-lived systems where a
large process starts and allocates a large number of THPs at the same time.
Instead of using a shared counter, this patch adds another bit to the
pageblock flags called PG_migrate_skip. If a pageblock is scanned by
either migrate or free scanner and 0 pages were isolated, the pageblock
is marked to be skipped in the future. When scanning, this bit is checked
before any scanning takes place and the block skipped if set.
The main difficulty with a patch like this is "when to ignore the cached
information?" If it's ignored too often, the scanning rates will still
be excessive. If the information is too stale then allocations will fail
that might have otherwise succeeded. In this patch