mm/damon/stat: expose the current tuned aggregation interval

mm/damon/stat: expose the current tuned aggregation interval

Patch series "mm/damon/stat: expose auto-tuned intervals and non-idle
ages".

DAMON_STAT is intentionally providing limited information for easy
consumption of the information.  From production fleet level usages, below
limitations are found, though.

The aggregation interval of DAMON_STAT represents the granularity of the
memory_idle_ms_percentiles.  But the interval is auto-tuned and not
exposed to users, so users cannot know the granularity.

All memory regions of non-zero (positive) nr_accesses are treated as
having zero idle time.  A significant portion of production systems have
such zero idle time.  Hence breakdown of warm and hot data is nearly
impossible.

Make following changes to overcome the limitations.  Expose the auto-tuned
aggregation interval with a new parameter named aggr_interval_us.  Expose
the age of non-zero nr_accesses (how long >0 access frequency the region
retained) regions as a negative idle time.

This patch (of 2):

DAMON_STAT calculates the idle time for a region as the region's age
multiplied by the aggregation interval.  That is, the aggregation interval
is the granularity of the idle time.  Since the aggregation interval is
auto-tuned and not exposed to users, however, users cannot easily know in
what granularity the stat is made.  Expose the tuned aggregation interval
in microseconds via a new parameter, aggr_interval_us.

Link: https://lkml.kernel.org/r/20250916183127.65708-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20250916183127.65708-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>