Re: [PATCH v2 1/1] cpuidle-powernv : forced wakeup for stop states

From: Abhishek
Date: Wed Jun 19 2019 - 05:13:30 EST

Hi Nick,

Thanks for the review. Some replies below.

On 06/19/2019 09:53 AM, Nicholas Piggin wrote:
Abhishek Goel's on June 17, 2019 7:56 pm:
Currently, the cpuidle governors determine what idle state a idling CPU
should enter into based on heuristics that depend on the idle history on
that CPU. Given that no predictive heuristic is perfect, there are cases
where the governor predicts a shallow idle state, hoping that the CPU will
be busy soon. However, if no new workload is scheduled on that CPU in the
near future, the CPU may end up in the shallow state.

This is problematic, when the predicted state in the aforementioned
scenario is a shallow stop state on a tickless system. As we might get
stuck into shallow states for hours, in absence of ticks or interrupts.

To address this, We forcefully wakeup the cpu by setting the
decrementer. The decrementer is set to a value that corresponds with the
residency of the next available state. Thus firing up a timer that will
forcefully wakeup the cpu. Few such iterations will essentially train the
governor to select a deeper state for that cpu, as the timer here
corresponds to the next available cpuidle state residency. Thus, cpu will
eventually end up in the deepest possible state.

Signed-off-by: Abhishek Goel <huntbag@xxxxxxxxxxxxxxxxxx>

v1 : started as auto promotion logic for cpuidle states in generic
v2 : Removed timeout_needed and rebased the code to upstream kernel
v1 : New patch with name of forced wakeup started
v2 : Extending the forced wakeup logic for all states. Setting the
decrementer instead of queuing up a hrtimer to implement the logic.

drivers/cpuidle/cpuidle-powernv.c | 38 +++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)

diff --git a/drivers/cpuidle/cpuidle-powernv.c b/drivers/cpuidle/cpuidle-powernv.c
index 84b1ebe212b3..bc9ca18ae7e3 100644
--- a/drivers/cpuidle/cpuidle-powernv.c
+++ b/drivers/cpuidle/cpuidle-powernv.c
@@ -46,6 +46,26 @@ static struct stop_psscr_table stop_psscr_table[CPUIDLE_STATE_MAX] __read_mostly
static u64 default_snooze_timeout __read_mostly;
static bool snooze_timeout_en __read_mostly;
+static u64 forced_wakeup_timeout(struct cpuidle_device *dev,
+ struct cpuidle_driver *drv,
+ int index)
+ int i;
+ for (i = index + 1; i < drv->state_count; i++) {
+ struct cpuidle_state *s = &drv->states[i];
+ struct cpuidle_state_usage *su = &dev->states_usage[i];
+ if (s->disabled || su->disable)
+ continue;
+ return (s->target_residency + 2 * s->exit_latency) *
+ tb_ticks_per_usec;
+ }
+ return 0;
It would be nice to not have this kind of loop iteration in the
idle fast path. Can we add a flag or something to the idle state?
Currently, we do not have any callback notification or some feedback that
notifies the driver everytime some state is enabled/disabled. So we have
to parse everytime to get the next enabled state. Are you suggesting to
add something like next_enabled_state in cpuidle state structure itself
which will be updated when a state is enabled or disabled?
static u64 get_snooze_timeout(struct cpuidle_device *dev,
struct cpuidle_driver *drv,
int index)
@@ -144,8 +164,26 @@ static int stop_loop(struct cpuidle_device *dev,
struct cpuidle_driver *drv,
int index)
+ u64 dec_expiry_tb, dec, timeout_tb, forced_wakeup;
+ dec = mfspr(SPRN_DEC);
+ timeout_tb = forced_wakeup_timeout(dev, drv, index);
+ forced_wakeup = 0;
+ if (timeout_tb && timeout_tb < dec) {
+ forced_wakeup = 1;
+ dec_expiry_tb = mftb() + dec;
+ }
The compiler probably can't optimise away the SPR manipulations so try
to avoid them if possible.
Are you suggesting something like set_dec_before_idle?(in line with
what you have suggested to do after idle, reset_dec_after_idle)

+ if (forced_wakeup)
+ mtspr(SPRN_DEC, timeout_tb);
This should just be put in the above 'if'.
Fair point.

+ if (forced_wakeup)
+ mtspr(SPRN_DEC, dec_expiry_tb - mftb());
This will sometimes go negative and result in another timer interrupt.

It also breaks irq work (which can be set here by machine check I

May need to implement some timer code to do this for you.

static void reset_dec_after_idle(void)
u64 now;
u64 *next_tb;

if (test_irq_work_pending())
now = mftb;
next_tb = this_cpu_ptr(&decrementers_next_tb);

if (now >= *next_tb)
set_dec(*next_tb - now);
if (test_irq_work_pending())

Something vaguely like that. See timer_interrupt().
Ah, Okay. Will go through timer_interrupt().