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Date:   Thu, 25 Jun 2020 11:53:25 -0000
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Subject: [tip: sched/core] sched: Force the address order of each sched class
 descriptor
Cc:     "Steven Rostedt (VMware)" <rostedt@goodmis.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
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The following commit has been merged into the sched/core branch of tip:

Commit-ID:     590d69796346353878b275c5512c664e3f875f24
Gitweb:        https://git.kernel.org/tip/590d69796346353878b275c5512c664e3f875f24
Author:        Steven Rostedt (VMware) <rostedt@goodmis.org>
AuthorDate:    Thu, 19 Dec 2019 16:44:52 -05:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 25 Jun 2020 13:45:43 +02:00

sched: Force the address order of each sched class descriptor

In order to make a micro optimization in pick_next_task(), the order of the
sched class descriptor address must be in the same order as their priority
to each other. That is:

 &idle_sched_class < &fair_sched_class < &rt_sched_class <
 &dl_sched_class < &stop_sched_class

In order to guarantee this order of the sched class descriptors, add each
one into their own data section and force the order in the linker script.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/157675913272.349305.8936736338884044103.stgit@localhost.localdomain
---
 include/asm-generic/vmlinux.lds.h | 13 +++++++++++++
 kernel/sched/deadline.c           |  3 ++-
 kernel/sched/fair.c               |  3 ++-
 kernel/sched/idle.c               |  3 ++-
 kernel/sched/rt.c                 |  3 ++-
 kernel/sched/stop_task.c          |  3 ++-
 6 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index db600ef..2186d7b 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -109,6 +109,18 @@
 #endif
 
 /*
+ * The order of the sched class addresses are important, as they are
+ * used to determine the order of the priority of each sched class in
+ * relation to each other.
+ */
+#define SCHED_DATA				\
+	*(__idle_sched_class)			\
+	*(__fair_sched_class)			\
+	*(__rt_sched_class)			\
+	*(__dl_sched_class)			\
+	*(__stop_sched_class)
+
+/*
  * Align to a 32 byte boundary equal to the
  * alignment gcc 4.5 uses for a struct
  */
@@ -388,6 +400,7 @@
 	.rodata           : AT(ADDR(.rodata) - LOAD_OFFSET) {		\
 		__start_rodata = .;					\
 		*(.rodata) *(.rodata.*)					\
+		SCHED_DATA						\
 		RO_AFTER_INIT_DATA	/* Read only after init */	\
 		. = ALIGN(8);						\
 		__start___tracepoints_ptrs = .;				\
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index d4708e2..d9e7946 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2479,7 +2479,8 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p,
 	}
 }
 
-const struct sched_class dl_sched_class = {
+const struct sched_class dl_sched_class
+	__attribute__((section("__dl_sched_class"))) = {
 	.next			= &rt_sched_class,
 	.enqueue_task		= enqueue_task_dl,
 	.dequeue_task		= dequeue_task_dl,
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0424a0a..3365f6b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11122,7 +11122,8 @@ static unsigned int get_rr_interval_fair(struct rq *rq, struct task_struct *task
 /*
  * All the scheduling class methods:
  */
-const struct sched_class fair_sched_class = {
+const struct sched_class fair_sched_class
+	__attribute__((section("__fair_sched_class"))) = {
 	.next			= &idle_sched_class,
 	.enqueue_task		= enqueue_task_fair,
 	.dequeue_task		= dequeue_task_fair,
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 8d75ca2..f580629 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -453,7 +453,8 @@ static void update_curr_idle(struct rq *rq)
 /*
  * Simple, special scheduling class for the per-CPU idle tasks:
  */
-const struct sched_class idle_sched_class = {
+const struct sched_class idle_sched_class
+	__attribute__((section("__idle_sched_class"))) = {
 	/* .next is NULL */
 	/* no enqueue/yield_task for idle tasks */
 
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index f395ddb..6543d44 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2429,7 +2429,8 @@ static unsigned int get_rr_interval_rt(struct rq *rq, struct task_struct *task)
 		return 0;
 }
 
-const struct sched_class rt_sched_class = {
+const struct sched_class rt_sched_class
+	__attribute__((section("__rt_sched_class"))) = {
 	.next			= &fair_sched_class,
 	.enqueue_task		= enqueue_task_rt,
 	.dequeue_task		= dequeue_task_rt,
diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
index 3e50a6a..f4bbd54 100644
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -109,7 +109,8 @@ static void update_curr_stop(struct rq *rq)
 /*
  * Simple, special scheduling class for the per-CPU stop tasks:
  */
-const struct sched_class stop_sched_class = {
+const struct sched_class stop_sched_class
+	__attribute__((section("__stop_sched_class"))) = {
 	.next			= &dl_sched_class,
 
 	.enqueue_task		= enqueue_task_stop,