mm/damon/core: use better timer mechanisms selection threshold

Patch series "mm/damon: Trivial fixups and improvements".

This patchset contains trivial fixups and improvements for DAMON and its
kunit/kselftest tests.

This patch (of 11):

DAMON is using hrtimer if requested sleep time is <=100ms, while the
suggested threshold[1] is <=20ms.  This commit applies the threshold.

[1] Documentation/timers/timers-howto.rst

Link: https://lkml.kernel.org/r/20211201150440.1088-2-sj@kernel.org
Fixes: ee801b7d ("mm/damon/schemes: activate schemes based on a watermarks mechanism")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/damon/core.c

@@ -980,7 +980,8 @@ static unsigned long damos_wmark_wait_us(struct damos *scheme)
 
 static void kdamond_usleep(unsigned long usecs)
 {
-	if (usecs > 100 * 1000)
+	/* See Documentation/timers/timers-howto.rst for the thresholds */
+	if (usecs > 20 * USEC_PER_MSEC)
 		schedule_timeout_idle(usecs_to_jiffies(usecs));
 	else
 		usleep_idle_range(usecs, usecs + 1);