Commit-ID: df55f462b905f3b2d40ec3fb865891382a6ebfb1
Gitweb: http://git.kernel.org/tip/df55f462b905f3b2d40ec3fb865891382a6ebfb1
Author: Gaurav Jindal (Gaurav Jindal) <Gaurav.Jindal@xxxxxxxxxxxxxx>
AuthorDate: Thu, 12 May 2016 10:13:33 +0000
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 3 Jun 2016 09:18:56 +0200
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 <gaurav.jindal@xxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Sanjeev Yadav <sanjeev.yadav@xxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20160512101330.GA488@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/sched/idle.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index bd12c6c..db4ff7c 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -201,6 +201,8 @@ exit_idle:
*/
static void cpu_idle_loop(void)
{
+ int cpu = smp_processor_id();
+
while (1) {
/*
* If the arch has a polling bit, we maintain an invariant:
@@ -219,7 +221,7 @@ static void cpu_idle_loop(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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