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Subject[PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt_curr()
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In kernel/sched/Makefile files, describing different sched classes, already
go in the order from the lowest priority class to the highest priority class:

idle.o fair.o rt.o deadline.o stop_task.o

The documentation of GNU linker says, that section appears in the order
they are seen during link time (see [1]):

>Normally, the linker will place files and sections matched by wildcards
>in the order in which they are seen during the link. You can change this
>by using the SORT keyword, which appears before a wildcard pattern
>in parentheses (e.g., SORT(.text*)).

So, we may expect const variables from idle.o will go before ro variables
from fair.o in RO_DATA section, while ro variables from fair.o will go
before ro variables from rt.o, etc.

(Also, it looks like the linking order is already used in kernel, e.g.
in drivers/md/Makefile)

Thus, we may introduce an optimization based on xxx_sched_class addresses
in these two hot scheduler functions: pick_next_task() and check_preempt_curr().

One more result of the patch is that size of object file becomes a little
less (excluding added BUG_ON(), which goes in __init section):

$size kernel/sched/core.o
text data bss dec hex filename
before: 66446 18957 676 86079 1503f kernel/sched/core.o
after: 66398 18957 676 86031 1500f kernel/sched/core.o

[1] https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/sections.html

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
kernel/sched/Makefile | 2 ++
kernel/sched/core.c | 24 +++++++++---------------
2 files changed, 11 insertions(+), 15 deletions(-)

diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index 5fc9c9b70862..f78f177c660a 100644
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -23,6 +23,8 @@ CFLAGS_core.o := $(PROFILING) -fno-omit-frame-pointer
endif

obj-y += core.o loadavg.o clock.o cputime.o
+# Order is significant: a more priority class xxx is described by variable
+# xxx_sched_class with a bigger address. See BUG_ON() in sched_init().
obj-y += idle.o fair.o rt.o deadline.o
obj-y += wait.o wait_bit.o swait.o completion.o

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 15508c202bf5..befdd7158b27 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1416,20 +1416,10 @@ static inline void check_class_changed(struct rq *rq, struct task_struct *p,

void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
{
- const struct sched_class *class;
-
- if (p->sched_class == rq->curr->sched_class) {
+ if (p->sched_class == rq->curr->sched_class)
rq->curr->sched_class->check_preempt_curr(rq, p, flags);
- } else {
- for_each_class(class) {
- if (class == rq->curr->sched_class)
- break;
- if (class == p->sched_class) {
- resched_curr(rq);
- break;
- }
- }
- }
+ else if (p->sched_class > rq->curr->sched_class)
+ resched_curr(rq);

/*
* A queue event has occurred, and we're going to schedule. In
@@ -3914,8 +3904,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
* higher scheduling class, because otherwise those loose the
* opportunity to pull in more work from other CPUs.
*/
- if (likely((prev->sched_class == &idle_sched_class ||
- prev->sched_class == &fair_sched_class) &&
+ if (likely(prev->sched_class <= &fair_sched_class &&
rq->nr_running == rq->cfs.h_nr_running)) {

p = pick_next_task_fair(rq, prev, rf);
@@ -6569,6 +6558,11 @@ void __init sched_init(void)
unsigned long ptr = 0;
int i;

+ BUG_ON(&idle_sched_class > &fair_sched_class ||
+ &fair_sched_class > &rt_sched_class ||
+ &rt_sched_class > &dl_sched_class ||
+ &dl_sched_class > &stop_sched_class);
+
wait_bit_init();

#ifdef CONFIG_FAIR_GROUP_SCHED

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