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 <hunt...@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
