From: SeongJae Park <redacted>

DAMON-based operation schemes need to be manually turned on and off.  In
some use cases, however, the condition for turning a scheme on and off
would depend on the system's situation.  For example, schemes for
proactive pages reclamation would need to be turned on when some memory
pressure is detected, and turned off when the system has enough free
memory.

For easier control of schemes activation based on the system situation,
this commit introduces a watermarks-based mechanism.  The client can
describe the watermark metric (e.g., amount of free memory in the
system), watermark check interval, and three watermarks, namely high,
mid, and low.  If the scheme is deactivated, it only gets the metric and
compare that to the three watermarks for every check interval.  If the
metric is higher than the high watermark, the scheme is deactivated.  If
the metric is between the mid watermark and the low watermark, the
scheme is activated.  If the metric is lower than the low watermark, the
scheme is deactivated again.  This is to allow users fall back to
traditional page-granularity mechanisms.

Signed-off-by: SeongJae Park <redacted>
---
 include/linux/damon.h | 52 +++++++++++++++++++++++++-
 mm/damon/core.c       | 87 ++++++++++++++++++++++++++++++++++++++++++-
 mm/damon/dbgfs.c      |  5 ++-
 3 files changed, 141 insertions(+), 3 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 565f49d8ba44..2edd84e98056 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h

@@ -127,6 +127,45 @@ struct damos_speed_limit {
 	unsigned int min_score;
 };

 static void set_kdamond_stop(struct damon_ctx *ctx)
 {
 	mutex_lock(&ctx->kdamond_lock);

@@ -904,6 +982,13 @@ static int kdamond_fn(void *data)
 	sz_limit = damon_region_sz_limit(ctx);
 
 	while (!kdamond_need_stop(ctx)) {
+		unsigned long wmark_wait_us = kdamond_wmark_wait_us(ctx);
+
+		if (wmark_wait_us) {
+			usleep_range(wmark_wait_us, wmark_wait_us + 1);
+			continue;
+		}