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

From: Abhishek
Date: Tue Jul 09 2019 - 01:33:26 EST

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>

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.
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);
+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())


+ 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.