[Patch]cpuidle: Save current cpu as local variable instead of calling smp_processor_id() in loop

[Gaurav Jindal (Gaurav Jindal)]

Currently, smp_processor_id() is used to fetch the current cpu in
cpu_idle_loop. Everytime the idle thread runs, it fetches current cpu
using smp_processor_id().
For idle thread which is per cpu, current cpu is constant and cannot
change at runtime. So moving the smp_processor_id() before the loop
saves execution cycles/time in loop.
With the patch, assembly code(on x-86 and ARM64) to be executed in loop
is reduced.
X-86 architecture:
Before patch(execution in loop):
148:    0f ae e8                lfence
14b:    65 8b 04 25 00 00 00 00 mov %gs:0x0,%eax
152:    00
153:    89 c0                   mov %eax,%eax
155:    49 0f a3 04 24          bt %rax,(%r12)

After patch(execution in loop):
150:    0f ae e8                lfence
153:    4d 0f a3 34 24          bt %r14,(%r12)

For ARM64:
Before patch(execution in loop):
168:    d5033d9f        dsb     ld
16c:    b9405661        ldr     w1,[x19,#84]
170:    1100fc20        add     w0,w1,#0x3f
174:    6b1f003f        cmp     w1,wzr
178:    1a81b000        csel    w0,w0,w1,lt
17c:    130c7000        asr     w0,w0,#6
180:    937d7c00        sbfiz   x0,x0,#3,#32
184:    f8606aa0        ldr     x0,[x21,x0]
188:    9ac12401        lsr     x1,x0,x1
18c:    36000e61        tbz     w1,#0,358

After patch(execution in loop):
1a8:    d50339df        dsb     ld
1ac:    f8776ac0        ldr     x0,[x22,x23]
ab0:    ea18001f        tst     x0,x24
1b4:    54000ea0        b.eq    388

Further observance on ARM64  for 4 seconds shows that cpu_idle_loop is
called 8672 times. Shifting the code will save instructions executed in
loop and eventually time as well.

Signed-off-by: Gaurav Jindal<gaurav.jindal@xxxxxxxxxxxxxx>
Reviewed-by: Sanjeev Yadav<sanjeev.yadav@xxxxxxxxxxxxxx>

---

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 1214f0a..82698e5 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -185,6 +185,8 @@ exit_idle:
  */
 static void cpu_idle_loop(void)
 {
+	int cpu_id;

+	cpu_id = smp_processor_id();
 	while (1) {
 		/*
 		 * If the arch has a polling bit, we maintain an invariant:
@@ -202,7 +204,7 @@ static void cpu_idle_loop(void)
 			check_pgt_cache();
 			rmb();

-			if (cpu_is_offline(smp_processor_id()))
+			if (cpu_is_offline(cpu_id))
                                arch_cpu_idle_dead();

 			local_irq_disable();

--