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

[SeongJae Park <sj@kernel.org>]

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 | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index 99ba346f9e325..510c593a313b8 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 +269,13 @@ 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 +285,7 @@ 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