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

Peter Zijlstra
2017-07-11T09:42:08Z

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?