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.jin...@spreadtrum.com>
Reviewed-by: Sanjeev Yadav<sanjeev.ya...@spreadtrum.com>
---
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();
--
