On 2017/7/12 13:03, Paul E. McKenney wrote: > On Wed, Jul 12, 2017 at 11:19:59AM +0800, Li, Aubrey wrote: >> On 2017/7/12 2:11, Paul E. McKenney wrote: >>> On Tue, Jul 11, 2017 at 06:33:55PM +0200, Frederic Weisbecker wrote: >>>> On Tue, Jul 11, 2017 at 05:58:47AM -0700, Paul E. McKenney wrote: >>>>> On Mon, Jul 10, 2017 at 09:38:34AM +0800, Aubrey Li wrote: >>>>>> From: Aubrey Li <aubrey...@linux.intel.com> >>>>>> >>>>>> The system will enter a fast idle loop if the predicted idle period >>>>>> is shorter than the threshold. >>>>>> --- >>>>>> kernel/sched/idle.c | 9 ++++++++- >>>>>> 1 file changed, 8 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c >>>>>> index cf6c11f..16a766c 100644 >>>>>> --- a/kernel/sched/idle.c >>>>>> +++ b/kernel/sched/idle.c >>>>>> @@ -280,6 +280,8 @@ static void cpuidle_generic(void) >>>>>> */ >>>>>> static void do_idle(void) >>>>>> { >>>>>> + unsigned int predicted_idle_us; >>>>>> + unsigned int short_idle_threshold = jiffies_to_usecs(1) / 2; >>>>>> /* >>>>>> * If the arch has a polling bit, we maintain an invariant: >>>>>> * >>>>>> @@ -291,7 +293,12 @@ static void do_idle(void) >>>>>> >>>>>> __current_set_polling(); >>>>>> >>>>>> - cpuidle_generic(); >>>>>> + predicted_idle_us = cpuidle_predict(); >>>>>> + >>>>>> + if (likely(predicted_idle_us < short_idle_threshold)) >>>>>> + cpuidle_fast(); >>>>> >>>>> What if we get here from nohz_full usermode execution? In that >>>>> case, if I remember correctly, the scheduling-clock interrupt >>>>> will still be disabled, and would have to be re-enabled before >>>>> we could safely invoke cpuidle_fast(). >>>>> >>>>> Or am I missing something here? >>>> >>>> That's a good point. It's partially ok because if the tick is needed >>>> for something specific, it is not entirely stopped but programmed to that >>>> deadline. >>>> >>>> Now there is some idle specific code when we enter dynticks-idle. See >>>> tick_nohz_start_idle(), tick_nohz_stop_idle(), >>>> sched_clock_idle_wakeup_event() >>>> and some subsystems that react differently when we enter dyntick idle >>>> mode (scheduler_tick_max_deferment) so the tick may need a reevaluation. >>>> >>>> For now I'd rather suggest that we treat full nohz as an exception case >>>> here >>>> and do: >>>> >>>> if (!tick_nohz_full_cpu(smp_processor_id()) && >>>> likely(predicted_idle_us < short_idle_threshold)) >>>> cpuidle_fast(); >>>> >>>> Ugly but safer! >>> >>> Works for me! >> >> I guess who enabled full nohz(for example the financial guys who need the >> system >> response as fast as possible) does not like this compromise, ;) > > And some HPC guys and some real-time guys with CPU-bound real-time > processing, so there are likely quite a few different views on this > compromise. > >> How about add rcu_idle enter/exit back only for full nohz case in fast idle? >> RCU idle >> is the only risky ops if removing them from fast idle path. Comparing to >> adding RCU >> idle back, going to normal idle path has more overhead IMHO. > > That might work, but I would need to see the actual patch. Frederic > Weisbecker should look at it as well. > Okay, let me address the first round of comments and deliver v2 soon.
Thanks, -Aubrey