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Subject[tip: sched/core] sched: Force the address order of each sched class descriptor
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,
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