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SubjectRe: [PATCH v2 1/1] cpuidle-powernv : forced wakeup for stop states
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@linux.vnet.ibm.com>
> ---
>
> Auto-promotion
> v1 : started as auto promotion logic for cpuidle states in generic
> driver
> v2 : Removed timeout_needed and rebased the code to upstream kernel
> Forced-wakeup
> 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?

> +
> 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.

> +
> + if (forced_wakeup)
> + mtspr(SPRN_DEC, timeout_tb);

This should just be put in the above 'if'.

> +
> power9_idle_type(stop_psscr_table[index].val,
> stop_psscr_table[index].mask);
> +
> + 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
believe.

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())
return;
now = mftb;
next_tb = this_cpu_ptr(&decrementers_next_tb);

if (now >= *next_tb)
return;
set_dec(*next_tb - now);
if (test_irq_work_pending())
set_dec(1);
}

Something vaguely like that. See timer_interrupt().

Thanks,
Nick

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