[RFC PATCH v1] sched/uclamp: Introduce SCHED_FLAG_RESET_UCLAMP_ON_FORK flag

[David Dai]

A userspace service may manage uclamp dynamically for individual tasks and
a child task will unintentionally inherit a pesudo-random uclamp setting.
This could result in the child task being stuck with a static uclamp value
that results in poor performance or poor power.

Using SCHED_FLAG_RESET_ON_FORK is too coarse for this usecase and will
reset other useful scheduler attributes. Adding a
SCHED_FLAG_RESET_UCLAMP_ON_FORK will allow userspace to have finer control
over scheduler attributes of child processes.

Cc: Qais Yousef <qyousef@xxxxxxxxxx>
Cc: Quentin Perret <qperret@xxxxxxxxxx>
Cc: Saravana Kannan <saravanak@xxxxxxxxxx>
Signed-off-by: David Dai <davidai@xxxxxxxxxx>
---
 include/linux/sched.h            | 3 +++
 include/uapi/linux/sched.h       | 4 +++-
 kernel/sched/core.c              | 6 +++++-
 tools/include/uapi/linux/sched.h | 4 +++-
 4 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 63d242164b1a..b1676b9381f9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -885,6 +885,9 @@ struct task_struct {
 	unsigned			sched_reset_on_fork:1;
 	unsigned			sched_contributes_to_load:1;
 	unsigned			sched_migrated:1;
+#ifdef CONFIG_UCLAMP_TASK
+	unsigned			sched_reset_uclamp_on_fork:1;
+#endif
 
 	/* Force alignment to the next boundary: */
 	unsigned			:0;
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 3bac0a8ceab2..7515106e1f1a 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -132,12 +132,14 @@ struct clone_args {
 #define SCHED_FLAG_KEEP_PARAMS		0x10
 #define SCHED_FLAG_UTIL_CLAMP_MIN	0x20
 #define SCHED_FLAG_UTIL_CLAMP_MAX	0x40
+#define SCHED_FLAG_RESET_UCLAMP_ON_FORK	0x80
 
 #define SCHED_FLAG_KEEP_ALL	(SCHED_FLAG_KEEP_POLICY | \
 				 SCHED_FLAG_KEEP_PARAMS)
 
 #define SCHED_FLAG_UTIL_CLAMP	(SCHED_FLAG_UTIL_CLAMP_MIN | \
-				 SCHED_FLAG_UTIL_CLAMP_MAX)
+				 SCHED_FLAG_UTIL_CLAMP_MAX | \
+				 SCHED_FLAG_RESET_UCLAMP_ON_FORK)
 
 #define SCHED_FLAG_ALL	(SCHED_FLAG_RESET_ON_FORK	| \
 			 SCHED_FLAG_RECLAIM		| \
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0d18c3969f90..f2d5f7911855 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1943,6 +1943,10 @@ static void __setscheduler_uclamp(struct task_struct *p,
 		uclamp_se_set(&p->uclamp_req[UCLAMP_MAX],
 			      attr->sched_util_max, true);
 	}
+
+	p->sched_reset_uclamp_on_fork = !!(attr->sched_flags &
+				     SCHED_FLAG_RESET_UCLAMP_ON_FORK);
+
 }
 
 static void uclamp_fork(struct task_struct *p)
@@ -1956,7 +1960,7 @@ static void uclamp_fork(struct task_struct *p)
 	for_each_clamp_id(clamp_id)
 		p->uclamp[clamp_id].active = false;
 
-	if (likely(!p->sched_reset_on_fork))
+	if (likely(!p->sched_reset_on_fork && !p->sched_reset_uclamp_on_fork))
 		return;
 
 	for_each_clamp_id(clamp_id) {
diff --git a/tools/include/uapi/linux/sched.h b/tools/include/uapi/linux/sched.h
index 3bac0a8ceab2..d52c59a2e0d0 100644
--- a/tools/include/uapi/linux/sched.h
+++ b/tools/include/uapi/linux/sched.h
@@ -132,12 +132,14 @@ struct clone_args {
 #define SCHED_FLAG_KEEP_PARAMS		0x10
 #define SCHED_FLAG_UTIL_CLAMP_MIN	0x20
 #define SCHED_FLAG_UTIL_CLAMP_MAX	0x40
+#define SCHED_FLAG_RESET_UCLAMP_ON_FORK 0x80
 
 #define SCHED_FLAG_KEEP_ALL	(SCHED_FLAG_KEEP_POLICY | \
 				 SCHED_FLAG_KEEP_PARAMS)
 
 #define SCHED_FLAG_UTIL_CLAMP	(SCHED_FLAG_UTIL_CLAMP_MIN | \
-				 SCHED_FLAG_UTIL_CLAMP_MAX)
+				 SCHED_FLAG_UTIL_CLAMP_MAX | \
+				 SCHED_FLAG_RESET_UCLAMP_ON_FORK)
 
 #define SCHED_FLAG_ALL	(SCHED_FLAG_RESET_ON_FORK	| \
 			 SCHED_FLAG_RECLAIM		| \
-- 
2.40.0.634.g4ca3ef3211-goog