Re: [PATCH v3 1/3] cpuidle-powernv : forced wakeup for stop states

[Nicholas Piggin <npiggin@gmail.com>]

Abhishek Goel's on July 4, 2019 7:18 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.
>  v3 : Cleanly handle setting/resetting of decrementer so as to not break
> irq work 
> 
>  arch/powerpc/include/asm/time.h   |  2 ++
>  arch/powerpc/kernel/time.c        | 40 +++++++++++++++++++++++++++++++
>  drivers/cpuidle/cpuidle-powernv.c | 32 +++++++++++++++++++++++++
>  3 files changed, 74 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
> index 54f4ec1f9..a3bd4f3c0 100644
> --- a/arch/powerpc/include/asm/time.h
> +++ b/arch/powerpc/include/asm/time.h
> @@ -188,6 +188,8 @@ static inline unsigned long tb_ticks_since(unsigned long tstamp)
>  extern u64 mulhdu(u64, u64);
>  #endif
>  
> +extern int set_dec_before_idle(u64 timeout);
> +extern void reset_dec_after_idle(void);
>  extern void div128_by_32(u64 dividend_high, u64 dividend_low,
>  			 unsigned divisor, struct div_result *dr);
>  
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 694522308..814de3469 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -576,6 +576,46 @@ void arch_irq_work_raise(void)
>  
>  #endif /* CONFIG_IRQ_WORK */
>  
> +/*
> + * Returns 1 if we have reprogrammed the decrementer for idle.
> + * Returns 0 if the decrementer is unchanged.
> + */
> +int set_dec_before_idle(u64 timeout)
> +{
> +	u64 *next_tb = this_cpu_ptr(&decrementers_next_tb);
> +	u64 now = get_tb_or_rtc();
> +
> +	/*
> +	 * Ensure that the timeout is at least one microsecond
> +	 * before the current decrement value. Else, we will
> +	 * unnecesarily wakeup again within a microsecond.
> +	 */
> +	if (now + timeout + 512 > *next_tb)

I would pass this 512 in as a parameter and put the comment in the
idle code. Timer code does not know/care.

Maybe return bool and call it try_set_dec_before_idle.

> +		return 0;
> +
> +	set_dec(timeout);

This needs to have

  if (test_irq_work_pending())
      set_dec(1);

here AFAIKS

> +
> +	return 1;
> +}
> +
> +void reset_dec_after_idle(void)
> +{
> +	u64 now;
> +	u64 *next_tb;
> +
> +	if (test_irq_work_pending())
> +		return;
> +
> +	now = get_tb_or_rtc();
> +	next_tb = this_cpu_ptr(&decrementers_next_tb);
> +	if (now >= *next_tb)
> +		return;

Are you sure it's okay to escape early in this case?

Thanks,
Nick