sched: idle: Reenable sched tick for cpuidle request

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  • sched: idle: Reenable sched tick for cpuidle request
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Leo Yan Aug. 9, 2018, 5:47 a.m.
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.

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.

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(-)

-- 
2.7.4

Comments

Leo Yan Aug. 9, 2018, 6:57 a.m. | #1
On Thu, Aug 09, 2018 at 01:47:27PM +0800, 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.

> 

> 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.

> 

> 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


I found the CPU6 data in upper table is flaw when I read this again,
CPU6 has no any ftrace event for idle entering/exiting from the start
testing, both two runs have the same issue.  so the result is not
reliable for CPU6.

Retested this case and at the beginning to wake up all CPUs so we can
have sane idle ftrace events.  Below is result, the conclusion is: CPU7
has improvement for staying in deepest state and there have no
regression on other CPUs.

       Without patches         With patches         Difference
     --------------------  --------------------  ----------------------
CPU    C0     C1      C2     C0     C1      C2      C0      C1      C2
0    0.000  0.021   9.837  0.000  0.022   9.919  +0.000  +0.000  +0.081
1    0.000  0.003  10.034  0.028  0.000   9.983  +0.028  -0.003  -0.051
2    0.023  0.031   9.963  0.007  0.019   9.986  -0.016  -0.011  +0.023
3    0.028  0.003   9.976  0.000  0.008  10.006  -0.027  +0.005  +0.030
4    0.052  0.000   9.971  0.023  0.000   9.994  -0.028  +0.000  +0.022
5    0.027  0.000  10.002  0.024  0.000   9.996  -0.002  +0.000  -0.006
6    0.013  0.000  10.018  0.025  0.000   9.992  +0.011  +0.000  -0.025
7    1.766  8.041   0.043  1.981  0.030   7.872  +0.214  -8.011  +7.829
All  1.912  8.101  69.847  2.092  0.081  77.752  +0.180  -8.020  +7.905


Another important dependency should to mention, we also need another
prerequisite patch "cpuidle: menu: Correct the criteria for stopping
tick" [1] for the testing, if without this patch, the idle governor
will select shallow state in idle loop but it will not tell idle loop
to reenable tick:

'expected_interval' is always be clamped to
min(TICK_USEC, ktime_to_us(delta_next)) [2] when tick is stopped,
thus 'expected_interval' is assigned to TICK_USEC at the last
time when the CPU enter idle state and without timer event, this results
in it cannot meet condition 'expected_interval < TICK_USEC' [3] for
enabling tick. We need rely on the dependent patch to set tick enabling
flag '*stop_tick = false' for shallow states.

[1] https://lkml.org/lkml/2018/8/7/407
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/cpuidle/governors/menu.c?h=v4.18-rc8#n358
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/cpuidle/governors/menu.c?h=v4.18-rc8#n407

> 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();

> +

>  			tick_nohz_idle_retain_tick();

> +		}

>  

>  		rcu_idle_enter();

>  

> -- 

> 2.7.4

>
Peter Zijlstra Aug. 9, 2018, 10:45 a.m. | #2
On Thu, Aug 09, 2018 at 01:47:27PM +0800, Leo Yan wrote:
> 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();

> +


I suspect you want an 'else' here. restart_tick already calls
timer_clear_idle().

>  			tick_nohz_idle_retain_tick();

> +		}

>  


However, I would rather we stuff all this into retain_tick.
Leo Yan Aug. 9, 2018, 11:17 a.m. | #3
On Thu, Aug 09, 2018 at 12:45:49PM +0200, Peter Zijlstra wrote:
> On Thu, Aug 09, 2018 at 01:47:27PM +0800, Leo Yan wrote:

> > 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();

> > +

> 

> I suspect you want an 'else' here. restart_tick already calls

> timer_clear_idle().


No, from the testing I found must call retain_tick, otherwise the
kernel compliants the warning from tick_nohz_idle_exit() when exit
from idle state:

        WARN_ON_ONCE(ts->timer_expires_base);

> >  			tick_nohz_idle_retain_tick();

> > +		}

> >  

> 

> However, I would rather we stuff all this into retain_tick.


Ah, yes; I tested below change and it also have same improvement for
idle state with my preivous change; please review if it's okay?

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index da9455a..fd2bfad 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -962,6 +962,10 @@ void tick_nohz_idle_stop_tick(void)
 
 void tick_nohz_idle_retain_tick(void)
 {
+       /* Restart the tikc if it has been stopped yet. */
+       if (tick_nohz_tick_stopped())
+               tick_nohz_idle_restart_tick();
+
        tick_nohz_retain_tick(this_cpu_ptr(&tick_cpu_sched));
        /*
         * Undo the effect of get_next_timer_interrupt() called from

Thanks,
Leo Yan
Leo Yan Aug. 10, 2018, 5:53 a.m. | #4
On Thu, Aug 09, 2018 at 11:31:46PM +0200, Rafael J . Wysocki wrote:

[...]

> > >> And I really would prefer to avoid restarting the tick here, because

> > >> it is overhead and quite likely unnecessary.

> > >

> > > I understand the logic when read the code, actually I did some experiments

> > > on the function menu_select(), in menu_select() function it discards the

> > > consideration for typical pattern interval and it also tries to avoid to

> > > enable tick and select more shallow state at the bottom of function.  So I

> > > agree that in the middle of idles it's redundant to reenable tick and the

> > > code is careful thought.

> > >

> > > But this patch tries to rescue the case at the last time the CPU enter one

> > > shallow idle state but without wake up event.

> > 

> > It is better to avoid entering a shallow state IMO.  Let me think

> > about that a bit.

> 

> The simple change below should address this issue and I don't quite see

> what it can break.  It may cause deeper idle states to be selected with

> the tick already stopped, but that really shouldn't be problematic, as

> (since the tick has been stopped) there are no strict latency constraints,

> so even if there is an early wakeup, we should be able to tolerate the

> extra latency just fine.

> 

> ---

>  drivers/cpuidle/governors/menu.c |   10 ++++------

>  1 file changed, 4 insertions(+), 6 deletions(-)

> 

> Index: linux-pm/drivers/cpuidle/governors/menu.c

> ===================================================================

> --- linux-pm.orig/drivers/cpuidle/governors/menu.c

> +++ linux-pm/drivers/cpuidle/governors/menu.c

> @@ -349,14 +349,12 @@ static int menu_select(struct cpuidle_dr

>  		 * If the tick is already stopped, the cost of possible short

>  		 * idle duration misprediction is much higher, because the CPU

>  		 * may be stuck in a shallow idle state for a long time as a

> -		 * result of it.  In that case say we might mispredict and try

> -		 * to force the CPU into a state for which we would have stopped

> -		 * the tick, unless a timer is going to expire really soon

> -		 * anyway.

> +		 * result of it.  In that case say we might mispredict and use

> +		 * the known time to the closest timer event for the idle state

> +		 * selection.

>  		 */

>  		if (data->predicted_us < TICK_USEC)

> -			data->predicted_us = min_t(unsigned int, TICK_USEC,

> -						   ktime_to_us(delta_next));

> +			data->predicted_us = ktime_to_us(delta_next);


I did the testing on this, but above change cannot really resolve the
issue, it misses to handle the case if 'data->predicted_us > TICK_USEC';
if the prediction is longer than TICK_USEC, e.g. data->predicted_us is
2ms, TICK_USEC=1ms;  for this case the deepest state will not be
chosen and if the data->predicted_us is decided by typical pattern
value but not the closest timer, finally the CPU still might stay in
shallow state for long time.

Actually in the CPU idle loop with the tick is stopped, I think we
should achieve two targets:
- Ensure the CPU can enter the deepest idle state at the last time it
  runs into into idle;
- In the middle of idles, we will not reenable the tick at all; though
  the idle states can be chosen a shallow state for short prediction;

To achieve the first target, we need to define what's the possible
case the CPU might stay into shallow state but cannot be waken up in
short time; so for this purpose it's pointeless to compare the value
between 'data->predicted_us' and TICK_USEC, so I'd like to check if
the next timer event is reliable to wake up CPU in short time, this
can be finished by comparison between 'ktime_to_us(delta_next)' with
maximum target residency;

For the second target, we should not enable the tick again in the idle
loop after the tick is stopped, whatever the governor choose any idle
state.

So how about below changes?  I did some verify on this.

diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 30ab759..e2de7c2 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -351,18 +351,21 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 	data->predicted_us = min(data->predicted_us, expected_interval);
 
 	if (tick_nohz_tick_stopped()) {
+		unsigned int delta_next_us = ktime_to_us(delta_next);
+
 		/*
 		 * If the tick is already stopped, the cost of possible short
 		 * idle duration misprediction is much higher, because the CPU
 		 * may be stuck in a shallow idle state for a long time as a
-		 * result of it.  In that case say we might mispredict and try
-		 * to force the CPU into a state for which we would have stopped
-		 * the tick, unless a timer is going to expire really soon
-		 * anyway.
+		 * result of it.  If the next timer event is later than the
+		 * maximum target residency, this means the timer event is not
+		 * trusted to wake up CPU in short term and the typical pattern
+		 * interval or other factors might lead to a shallow state, in
+		 * that case say we might mispredict and use the known time to
+		 * the closest timer event for the idle state selection.
 		 */
-		if (data->predicted_us < TICK_USEC)
-			data->predicted_us = min_t(unsigned int, TICK_USEC,
-						   ktime_to_us(delta_next));
+		if (delta_next_us >= drv->states[drv->state_count-1].target_residency)
+			data->predicted_us = delta_next_us;
 	} else {
 		/*
 		 * Use the performance multiplier and the user-configurable
@@ -410,12 +413,12 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 	 * expected idle duration is shorter than the tick period length.
 	 */
 	if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
-	    expected_interval < TICK_USEC) {
+	    (!tick_nohz_tick_stopped() && expected_interval < TICK_USEC)) {
 		unsigned int delta_next_us = ktime_to_us(delta_next);
 
 		*stop_tick = false;
 
-		if (!tick_nohz_tick_stopped() && idx > 0 &&
+		if (idx > 0 &&
 		    drv->states[idx].target_residency > delta_next_us) {
 			/*
 			 * The tick is not going to be stopped and the target

Patch

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