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SubjectRe: [PATCH] sched: idle: Reenable sched tick for cpuidle request
On Thu, Aug 9, 2018 at 2:05 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> On Thursday, August 9, 2018 7:47:27 AM CEST Leo Yan wrote:
>> The idle loop stops tick by respecting the decision from cpuidle
>> framework, if the condition 'need_resched()' is false without any task
>> scheduling, the CPU keeps running in the loop in do_idle() and it has no
>> chance call tick_nohz_idle_exit() to enable the tick. This results in
>> the idle loop cannot reenable sched tick afterwards, if the idle
>> governor selects a shallow state, thus the powernightmares issue can
>> occur again.
>
> Well, there is code in the menu governor to avoid that.

So the governor is not expected to select a shallow state then, unless
it knows that there will be a timer interrupt (which is not a tick,
obviously), that will wake up the CPU early enough.

The menu governor works like that, but I agree that it should not
request the tick to be running then (which it does).

>> This issue can be easily reproduce with the case on Arm Hikey board: use
>> CPU0 to send IPI to CPU7, CPU7 receives the IPI and in the callback
>> function it start a hrtimer with 4ms, so the 4ms timer delta value can
>> let 'menu' governor to choose deepest state in the next entering idle
>> time. From then on, CPU7 restarts hrtimer with 1ms interval for total
>> 10 times, so this can utilize the typical pattern in 'menu' governor to
>> have prediction for 1ms duration, finally idle governor is easily to
>> select a shallow state, on Hikey board it usually is to select CPU off
>> state. From then on, CPU7 stays in this shallow state for long time
>> until there have other interrupts on it.
>
> And which means that the above-mentioned code misses this case.

And I don't really understand how this happens. :-/

If menu sees that the tick has been stopped, it sets
data->predicted_us to the minimum of TICK_USEC and
ktime_to_us(delta_next) and the latency requirements comes from PM QoS
(no interactivity boost). Thus the only case when it will say "do not
stop the tick" is when delta_next is below the tick period length, but
that's OK, because it means that there is a timer pending that much
time away, so it doesn't make sense to select a deeper idle state
then.

If there is a short-interval timer pending every time we go idle, it
doesn't matter that the tick is stopped really, because the other
timer will wake the CPU up anyway.

Have I missed anything?

>>
>> C2: cluster off; C1: CPU off
>>
>> Idle state: C2 C2 C2 C2 C2 C2 C2 C1
>> --------------------------------------------------------->
>> Interrupt: ^ ^ ^ ^ ^ ^ ^ ^ ^
>> IPI Timer Timer Timer Timer Timer Timer Timer Timer
>> 4ms 1ms 1ms 1ms 1ms 1ms 1ms 1ms
>>
>> To fix this issue, the idle loop needs to support reenabling sched tick.
>> This patch checks the conditions 'stop_tick' is false when the tick is
>> stopped, this condition indicates the cpuidle governor asks to reenable
>> the tick and we can use tick_nohz_idle_restart_tick() for this purpose.
>>
>> A synthetic case is used to to verify this patch, we use CPU0 to send
>> IPI to wake up CPU7 with 50ms interval, CPU7 generate a series hrtimer
>> events (the first interval is 4ms, then the sequential 10 timer events
>> are 1ms interval, same as described above). We do statistics for idle
>> states duration, the unit is second (s), the testing result shows the
>> C2 state (deepest state) staying time can be improved significantly for
>> CPU7 (+7.942s for 10s execution time on CPU7) and all CPUs wide
>> (+13.360s for ~80s of all CPUs execution time).
>>
>> Without patches With patches Difference
>> -------------------- -------------------- -----------------------
>> CPU C0 C1 C2 C0 C1 C2 C0 C1 C2
>> 0 0.000 0.027 9.941 0.055 0.038 9.700 +0.055 +0.010 -0.240
>> 1 0.045 0.000 9.964 0.019 0.000 9.943 -0.026 +0.000 -0.020
>> 2 0.002 0.003 10.007 0.035 0.053 9.916 +0.033 +0.049 -0.090
>> 3 0.000 0.023 9.994 0.024 0.246 9.732 +0.024 +0.222 -0.261
>> 4 0.032 0.000 9.985 0.015 0.007 9.993 -0.016 +0.007 +0.008
>> 5 0.001 0.000 9.226 0.039 0.000 9.971 +0.038 +0.000 +0.744
>> 6 0.000 0.000 0.000 0.036 0.000 5.278 +0.036 +0.000 +5.278
>> 7 1.894 8.013 0.059 1.509 0.026 8.002 -0.384 -7.987 +7.942
>> All 1.976 8.068 59.179 1.737 0.372 72.539 -0.239 -7.695 +13.360
>>
>> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
>> Cc: Vincent Guittot <vincent.guittot@linaro.org>
>> Signed-off-by: Leo Yan <leo.yan@linaro.org>
>> ---
>> kernel/sched/idle.c | 12 ++++++++++--
>> 1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
>> index 1a3e9bd..802286e 100644
>> --- a/kernel/sched/idle.c
>> +++ b/kernel/sched/idle.c
>> @@ -190,10 +190,18 @@ static void cpuidle_idle_call(void)
>> */
>> next_state = cpuidle_select(drv, dev, &stop_tick);
>>
>> - if (stop_tick)
>> + if (stop_tick) {
>> tick_nohz_idle_stop_tick();
>> - else
>> + } else {
>> + /*
>> + * The cpuidle framework says to not stop tick but
>> + * the tick has been stopped yet, so restart it.
>> + */
>> + if (tick_nohz_tick_stopped())
>> + tick_nohz_idle_restart_tick();
>
> You need an "else" here IMO as Peter said.

And I really would prefer to avoid restarting the tick here, because
it is overhead and quite likely unnecessary.

>> +
>> tick_nohz_idle_retain_tick();
>> + }
>>
>> rcu_idle_enter();
>>
>>
>
> Please CC cpuidle patches to linux-pm@vger.kernel.org, that helps a lot.

Thanks,
Rafael

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