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

[Abhishek]

Hi Nick,

Will post next version with the changes you have suggested.
There is a comment below.

On 07/07/2019 03:43 PM, Nicholas Piggin wrote:
> 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@xxxxxxxxxxxxxxxxxx>
> > ---
> >
> > 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?

Yeah, It looks safe. In power9_idle_type, we call irq_set_pending_from_srr1
which sets the irq_happened. If reason is IRQ_DEC, in __check_irq_replay,
decrementer_check_overflow will be called which will set dec to a positive
valid value.
Also, we typically disable MSR EE before entering stop. And if a decrementer
wakes us up, before we enable EE, check for pending interrupt will be done.
And we finally reset dec to a positive value before we set EE=1.
> Thanks,
> Nick

Thanks,
Abhishek