[PATCH for-v5.15.x 2/2] mm/damon/core: fix fake load reports due to uninterruptible sleeps

[SeongJae Park <sj@kernel.org>]

commit 70e9274805fccfd175d0431a947bfd11ee7df40e upstream.

Because DAMON sleeps in uninterruptible mode, /proc/loadavg reports fake
load while DAMON is turned on, though it is doing nothing.  This can
confuse users[1].  To avoid the case, this commit makes DAMON sleeps in
idle mode.

[1] https://lore.kernel.org/all/11868371.O9o76ZdvQC@natalenko.name/

Link: https://lkml.kernel.org/r/20211126145015.15862-3-sj@kernel.org
Fixes: 2224d8485492 ("mm: introduce Data Access MONitor (DAMON)")
Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: SeongJae Park <sj@kernel.org>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 mm/damon/core.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 30e9211f494a..7a4912d6e65f 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -357,6 +357,15 @@ int damon_start(struct damon_ctx **ctxs, int nr_ctxs)
 	return err;
 }
 
+static void kdamond_usleep(unsigned long usecs)
+{
+	/* See Documentation/timers/timers-howto.rst for the thresholds */
+	if (usecs > 20 * 1000)
+		schedule_timeout_idle(usecs_to_jiffies(usecs));
+	else
+		usleep_idle_range(usecs, usecs + 1);
+}
+
 /*
  * __damon_stop() - Stops monitoring of given context.
  * @ctx:	monitoring context
@@ -370,8 +379,7 @@ static int __damon_stop(struct damon_ctx *ctx)
 		ctx->kdamond_stop = true;
 		mutex_unlock(&ctx->kdamond_lock);
 		while (damon_kdamond_running(ctx))
-			usleep_range(ctx->sample_interval,
-					ctx->sample_interval * 2);
+			kdamond_usleep(ctx->sample_interval);
 		return 0;
 	}
 	mutex_unlock(&ctx->kdamond_lock);
@@ -670,7 +678,7 @@ static int kdamond_fn(void *data)
 				ctx->callback.after_sampling(ctx))
 			set_kdamond_stop(ctx);
 
-		usleep_range(ctx->sample_interval, ctx->sample_interval + 1);
+		kdamond_usleep(ctx->sample_interval);
 
 		if (ctx->primitive.check_accesses)
 			max_nr_accesses = ctx->primitive.check_accesses(ctx);
-- 
2.17.1