Re: [PATCH] sched: idle: Reenable sched tick for cpuidle request

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

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.

> 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.

> 
> 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.

> +
>  			tick_nohz_idle_retain_tick();
> +		}
>  
>  		rcu_idle_enter();
>  
> 

Please CC cpuidle patches to linux-pm@vger.kernel.org, that helps a lot.

Thanks,
Rafael