linux-kernel - [RFC PATCH 2/2] mm/damon/stat: expose negative idle time

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Message-Id: <20250910040022.168223-3-sj@kernel.org>
Date: Tue,  9 Sep 2025 21:00:22 -0700
From: SeongJae Park <sj@...nel.org>
To: 
Cc: SeongJae Park <sj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	damon@...ts.linux.dev,
	kernel-team@...a.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [RFC PATCH 2/2] mm/damon/stat: expose negative idle time

DAMON_STAT calculates the idle time of a region using the region's age
if the region's nr_accesses is zero.  If the nr_accesses value is
non-zero (positive), the idle time of the region becomes zero.

This means the users cannot know how warm and hot data is distributed,
using DAMON_STAT's memory_idle_ms_percentiles output.  The other stat,
namely estimated_memory_bandwidth, can help understanding how the
overall access temperature of the system is, but it is still very rough
information.  On production systems, actually, a significant portion of
the system memory is observed with zero idle time, and we cannot break
it down based on its internal hotness distribution.

Define the idle time of the region using its age, similar to those
having zero nr_accesses, but multiples '-1' to distinguish it.  And
expose that using the same parameter interface,
memory_idle_ms_percentiles.

Signed-off-by: SeongJae Park <sj@...nel.org>
---
 mm/damon/stat.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index ff1c076185e6..c33df0ade183 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -34,8 +34,8 @@ module_param(estimated_memory_bandwidth, ulong, 0400);
 MODULE_PARM_DESC(estimated_memory_bandwidth,
 		"Estimated memory bandwidth usage in bytes per second");

-static unsigned long memory_idle_ms_percentiles[101] __read_mostly = {0,};
-module_param_array(memory_idle_ms_percentiles, ulong, NULL, 0400);
+static long memory_idle_ms_percentiles[101] __read_mostly = {0,};
+module_param_array(memory_idle_ms_percentiles, long, NULL, 0400);
 MODULE_PARM_DESC(memory_idle_ms_percentiles,
 		"Memory idle time percentiles in milliseconds");

@@ -61,10 +61,10 @@ static void damon_stat_set_estimated_memory_bandwidth(struct damon_ctx *c)
 		MSEC_PER_SEC / c->attrs.aggr_interval;
 }

-static unsigned int damon_stat_idletime(const struct damon_region *r)
+static int damon_stat_idletime(const struct damon_region *r)
 {
 	if (r->nr_accesses)
-		return 0;
+		return -1 * (r->age + 1);
 	return r->age + 1;
 }

@@ -122,7 +122,7 @@ static void damon_stat_set_idletime_percentiles(struct damon_ctx *c)
 		while (next_percentile <= accounted_bytes * 100 / total_sz)
 			memory_idle_ms_percentiles[next_percentile++] =
 				damon_stat_idletime(region) *
-				c->attrs.aggr_interval / USEC_PER_MSEC;
+				(long)c->attrs.aggr_interval / USEC_PER_MSEC;
 	}
 	kfree(sorted_regions);
 }
-- 
2.39.5