From ca248c7e0422c367835ae83199e9b9284b5e63fb Mon Sep 17 00:00:00 2001 From: "Gaurav Jindal (Gaurav Jindal)" Date: Thu, 12 May 2016 10:13:33 +0000 Subject: sched/idle: Optimize the generic idle loop Currently, smp_processor_id() is used to fetch the current CPU in cpu_idle_loop(). Every time the idle thread runs, it fetches the current CPU using smp_processor_id(). Since the idle thread is per CPU, the current CPU is constant, so we can lift the load out of the loop, saving execution cycles/time in the loop. x86-64: 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) 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Sanjeev Yadav Cc: Linus Torvalds Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20160512101330.GA488@gauravjindalubtnb.del.spreadtrum.com Signed-off-by: Ingo Molnar --- kernel/sched/idle.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'kernel') diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index c378dc659..a0f285fb9 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -93,6 +93,9 @@ void __weak arch_cpu_idle(void) */ static void cpu_idle_loop(void) { + int cpu = smp_processor_id(); + + #ifdef CONFIG_MT_LOAD_BALANCE_PROFILER mt_lbprof_update_state(smp_processor_id(), MT_LBPROF_NO_TASK_STATE); #endif @@ -104,7 +107,7 @@ static void cpu_idle_loop(void) check_pgt_cache(); rmb(); - if (cpu_is_offline(smp_processor_id())) + if (cpu_is_offline(cpu)) arch_cpu_idle_dead(); #ifdef CONFIG_MT_LOAD_BALANCE_PROFILER -- cgit v1.2.3