[tip:sched/core] sched/idle: Micro-optimize the idle loop

[tip-bot for Cheng Jian <tipbot@zytor.com>]

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();
 		}