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Subject[tip:sched/core] sched/idle: Micro-optimize the idle loop
Commit-ID:  54b933c6c954a8b7b0c2b40a1c4d3f7279d11e22
Gitweb: https://git.kernel.org/tip/54b933c6c954a8b7b0c2b40a1c4d3f7279d11e22
Author: Cheng Jian <cj.chengjian@huawei.com>
AuthorDate: Wed, 25 Oct 2017 19:28:27 +0800
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 26 Oct 2017 08:31:29 +0200

sched/idle: Micro-optimize the idle loop

Move the loop-invariant calculation of 'cpu' in do_idle() out of the loop body,
because the current CPU is always constant.

This improves the generated code both on x86-64 and ARM64:

x86-64:

Before patch (execution in loop):
864: 0f ae e8 lfence
867: 65 8b 05 c2 38 f1 7e mov %gs:0x7ef138c2(%rip),%eax
86e: 89 c0 mov %eax,%eax
870: 48 0f a3 05 68 19 08 bt %rax,0x1081968(%rip)
877: 01

After patch (execution in loop):
872: 0f ae e8 lfence
875: 4c 0f a3 25 63 19 08 bt %r12,0x1081963(%rip)
87c: 01

ARM64:

Before patch (execution in loop):
c58: d5033d9f dsb ld
c5c: d538d080 mrs x0, tpidr_el1
c60: b8606a61 ldr w1, [x19,x0]
c64: 1100fc20 add w0, w1, #0x3f
c68: 7100003f cmp w1, #0x0
c6c: 1a81b000 csel w0, w0, w1, lt
c70: 13067c00 asr w0, w0, #6
c74: 93407c00 sxtw x0, w0
c78: f8607a80 ldr x0, [x20,x0,lsl #3]
c7c: 9ac12401 lsr x1, x0, x1
c80: 36000581 tbz w1, #0, d30 <do_idle+0x128>

After patch (execution in loop):
c84: d5033d9f dsb ld
c88: f9400260 ldr x0, [x19]
c8c: ea14001f tst x0, x20
c90: 54000580 b.eq d40 <do_idle+0x138>

Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
[ Rewrote the title and the changelog. ]
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: huawei.libin@huawei.com
Cc: xiexiuqi@huawei.com
Link: http://lkml.kernel.org/r/1508930907-107755-1-git-send-email-cj.chengjian@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/sched/idle.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index b2e8f0a..7dae9eb 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -209,6 +209,7 @@ exit_idle:
*/
static void do_idle(void)
{
+ int cpu = smp_processor_id();
/*
* If the arch has a polling bit, we maintain an invariant:
*
@@ -225,7 +226,7 @@ static void do_idle(void)
check_pgt_cache();
rmb();

- if (cpu_is_offline(smp_processor_id())) {
+ if (cpu_is_offline(cpu)) {
cpuhp_report_idle_dead();
arch_cpu_idle_dead();
}
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