Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

[Peter Zijlstra]

On Tue, Jul 11, 2017 at 12:40:06PM +0800, Li, Aubrey wrote:
> > On Mon, Jul 10, 2017 at 06:42:06PM +0200, Peter Zijlstra wrote:

> >> Data to indicate what hurts how much would be a very good addition to
> >> the Changelogs. Clearly you have some, you really should have shared.

> In the idle loop,
> 
> - quiet_vmstat costs 5562ns - 6296ns

Urgh, that thing is horrible, also I think its placed wrong. The comment
near that function says it should be called when we enter NOHZ.

Which suggests something like so:

---
 kernel/sched/idle.c      | 1 -
 kernel/time/tick-sched.c | 1 +
 2 files changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 6c23e30c0e5c..ef63adce0c9c 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -219,7 +219,6 @@ static void do_idle(void)
 	 */
 
 	__current_set_polling();
-	quiet_vmstat();
 	tick_nohz_idle_enter();
 
 	while (!need_resched()) {
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index c7a899c5ce64..eb0e9753db8f 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -787,6 +787,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 	if (!ts->tick_stopped) {
 		calc_load_nohz_start();
 		cpu_load_update_nohz_start();
+		quiet_vmstat();
 
 		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
 		ts->tick_stopped = 1;


> - tick_nohz_idle_enter costs 7058ns - 10726ns
> - tick_nohz_idle_exit costs 8372ns - 20850ns

Right, those are horrible expensive, but skipping them isn't 'hard', the
only tricky bit is finding a condition that makes sense.

See Mike's patch: https://patchwork.kernel.org/patch/2839221/

Combined with the above, and possibly a better condition, that should
get rid of most of this.

> - totally from arch_cpu_idle_enter entry to arch_cpu_idle_exit return costs
>   9122ns - 15318ns.
>   --In this period, rcu_idle_enter costs 1985ns - 2262ns, rcu_idle_exit costs
>     1813ns - 3507ns

Is that the POPF being painful? or something else?