mm/damon: prevent activated scheme from sleeping by deactivated schemes

In the DAMON, the minimum wait time of the schemes decides whether the
kernel wakes up 'kdamon_fn()'.  But since the minimum wait time is
initialized to zero, there are corner cases against the original
objective.

For example, if we have several schemes for one target, and if the wait
time of the first scheme is zero, the minimum wait time will set zero,
which means 'kdamond_fn()' should wake up to apply this scheme.
However, in the following scheme, wait time can be set to non-zero.
Thus, the mininum wait time will be set to non-zero, which can cause
sleeping this interval for 'kdamon_fn()' due to one deactivated last
scheme.

This commit prevents making DAMON monitoring inactive state due to other
deactivated schemes.

Link: https://lkml.kernel.org/r/20220330105302.32114-1-tome01@ajou.ac.kr
Signed-off-by: Jonghyeon Kim <tome01@ajou.ac.kr>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/damon/core.c

@@ -1019,12 +1019,15 @@ static int kdamond_wait_activation(struct damon_ctx *ctx)
 	struct damos *s;
 	unsigned long wait_time;
 	unsigned long min_wait_time = 0;
+	bool init_wait_time = false;
 
 	while (!kdamond_need_stop(ctx)) {
 		damon_for_each_scheme(s, ctx) {
 			wait_time = damos_wmark_wait_us(s);
-			if (!min_wait_time || wait_time < min_wait_time)
+			if (!init_wait_time || wait_time < min_wait_time) {
+				init_wait_time = true;
 				min_wait_time = wait_time;
+			}
 		}
 		if (!min_wait_time)
 			return 0;