[RFC PATCH v1.2 0/2] mm/damon/stat: add kdamond_pid parameter

[RFC PATCH v1.2 0/2] mm/damon/stat: add kdamond_pid parameter

[SeongJae Park]

DAMON_STAT doesn't provide the pid of its kdamond, unlike DAMON_RECLAIM
and DAMON_LRU_SORT.  This makes user-space management of DAMON_STAT
unnecessarily complicated.  Provide the information via a new parameter,
namely kdamond_pid, and document it.

Changes from RFC v1.1
- rfc v1.1: https://lore.kernel.org/20260414235912.98174-1-sj@kernel.org
- Close the parentheses of error handling block.
Changes from RFC
- rfc: https://lore.kernel.org/20260414053742.90296-1-sj@kernel.org
- Fix damon_kdamond_pid() failure handling.

SeongJae Park (2):
  mm/damon/stat: add a parameter for reading kdamond pid
  Docs/admin-guide/mm/damon/stat: document kdamond_pid parameter

 Documentation/admin-guide/mm/damon/stat.rst |  7 +++++++
 mm/damon/stat.c                             | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+)


base-commit: bf44f59d29186d80db01e4124a8ab23b3b235b32
-- 
2.47.3

[RFC PATCH v1.2 1/2] mm/damon/stat: add a parameter for reading kdamond pid

[SeongJae Park]

Knowing the pid of the kdamonds can help user-space management including
monitoring of DAMON's system resource consumption.  To make it easier,
DAMON_SYSFS, DAMON_RECLAIM and DAMON_LRU_SORT provide the pid
information.  DAMON_STAT is not providing it, though.  Expose the pid of
DAMON_STAT kdamond via a new read-only module parameter, namely
kdamond_pid.  This also makes DAMON modules usage more standardized,
because DAMON_RECLAIM and DAMON_LRU_SORT also provide the information
via their read-only parameters of the same name.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/stat.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index 99ba346f9e325..4901e9a7c3398 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -44,6 +44,15 @@ module_param(aggr_interval_us, ulong, 0400);
 MODULE_PARM_DESC(aggr_interval_us,
 		"Current tuned aggregation interval in microseconds");
 
+/*
+ * PID of the DAMON thread
+ *
+ * If DAMON_STAT is enabled, this becomes the PID of the worker thread.
+ * Else, -1.
+ */
+static int kdamond_pid __read_mostly = -1;
+module_param(kdamond_pid, int, 0400);
+
 static struct damon_ctx *damon_stat_context;
 
 static unsigned long damon_stat_last_refresh_jiffies;
@@ -260,6 +269,14 @@ static int damon_stat_start(void)
 		damon_stat_context = NULL;
 		return err;
 	}
+	kdamond_pid = damon_kdamond_pid(damon_stat_context);
+	if (kdamond_pid < 0) {
+		err = kdamond_pid;
+		kdamond_pid = -1;
+		damon_destroy_ctx(damon_stat_context);
+		damon_stat_context = NULL;
+		return err;
+	}
 
 	damon_stat_last_refresh_jiffies = jiffies;
 	call_control.data = damon_stat_context;
@@ -269,6 +286,7 @@ static int damon_stat_start(void)
 static void damon_stat_stop(void)
 {
 	damon_stop(&damon_stat_context, 1);
+	kdamond_pid = -1;
 	damon_destroy_ctx(damon_stat_context);
 	damon_stat_context = NULL;
 }
-- 
2.47.3

[RFC PATCH v1.2 2/2] Docs/admin-guide/mm/damon/stat: document kdamond_pid parameter

[SeongJae Park]

Update DAMON_STAT usage document for newly added kdamond_pid parameter.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/admin-guide/mm/damon/stat.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/admin-guide/mm/damon/stat.rst b/Documentation/admin-guide/mm/damon/stat.rst
index c4b14daeb2dd6..46c5dd96aa2ed 100644
--- a/Documentation/admin-guide/mm/damon/stat.rst
+++ b/Documentation/admin-guide/mm/damon/stat.rst
@@ -89,3 +89,10 @@ percentiles of the idle time values via this read-only parameter.  Reading the
 parameter returns 101 idle time values in milliseconds, separated by comma.
 Each value represents 0-th, 1st, 2nd, 3rd, ..., 99th and 100th percentile idle
 times.
+
+kdamond_pid
+-----------
+
+PID of the DAMON thread.
+
+If DAMON_STAT is enabled, this becomes the PID of the worker thread.  Else, -1.
-- 
2.47.3