mm/damon/core: use number of passed access sampling as a timer

DAMON sleeps for sampling interval after each sampling, and check if the
aggregation interval and the ops update interval have passed using
ktime_get_coarse_ts64() and baseline timestamps for the intervals.  That
design is for making the operations occur at deterministic timing
regardless of the time that spend for each work.  However, it turned out
it is not that useful, and incur not-that-intuitive results.

After all, timer functions, and especially sleep functions that DAMON uses
to wait for specific timing, are not necessarily strictly accurate.  It is
legal design, so no problem.  However, depending on such inaccuracies, the
nr_accesses can be larger than aggregation interval divided by sampling
interval.  For example, with the default setting (5 ms sampling interval
and 100 ms aggregation interval) we frequently show regions having
nr_accesses larger than 20.  Also, if the execution of a DAMOS scheme
takes a long time, next aggregation could happen before enough number of
samples are collected.  This is not what usual users would intuitively
expect.

Since access check sampling is the smallest unit work of DAMON, using the
number of passed sampling intervals as the DAMON-internal timer can easily
avoid these problems.  That is, convert aggregation and ops update
intervals to numbers of sampling intervals that need to be passed before
those operations be executed, count the number of passed sampling
intervals, and invoke the operations as soon as the specific amount of
sampling intervals passed.  Make the change.

Note that this could make a behavioral change to settings that using
intervals that not aligned by the sampling interval.  For example, if the
sampling interval is 5 ms and the aggregation interval is 12 ms, DAMON
effectively uses 15 ms as its aggregation interval, because it checks
whether the aggregation interval after sleeping the sampling interval. 
This change will make DAMON to effectively use 10 ms as aggregation
interval, since it uses 'aggregation interval / sampling interval *
sampling interval' as the effective aggregation interval, and we don't use
floating point types.  Usual users would have used aligned intervals, so
this behavioral change is not expected to make any meaningful impact, so
just make this change.

Link: https://lkml.kernel.org/r/20230914021523.60649-1-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 12 insertions, 2 deletions

diff --git a/include/linux/damon.h b/include/linux/damon.h
index ab3089de1478..9a32b8fd0bd3 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -524,8 +524,18 @@ struct damon_ctx {
 	struct damon_attrs attrs;
 
 /* private: internal use only */
-	struct timespec64 last_aggregation;
-	struct timespec64 last_ops_update;
+	/* number of sample intervals that passed since this context started */
+	unsigned long passed_sample_intervals;
+	/*
+	 * number of sample intervals that should be passed before next
+	 * aggregation
+	 */
+	unsigned long next_aggregation_sis;
+	/*
+	 * number of sample intervals that should be passed before next ops
+	 * update
+	 */
+	unsigned long next_ops_update_sis;
 
 /* public: */
 	struct task_struct *kdamond;