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 will end up in the shallow state.
Motivation ---------- In case of POWER, 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 even 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 and we won't get stuck in a shallow state for long duration. Experiment ---------- For earlier versions when this feature was meat to be only for shallow lite states, I performed experiments for three scenarios to collect some data. case 1 : Without this patch and without tick retained, i.e. in a upstream kernel, It would spend more than even a second to get out of stop0_lite. case 2 : With tick retained in a upstream kernel - Generally, we have a sched tick at 4ms(CONF_HZ = 250). Ideally I expected it to take 8 sched tick to get out of stop0_lite. Experimentally, observation was ========================================================= sample min max 99percentile 20 4ms 12ms 4ms ========================================================= It would take atleast one sched tick to get out of stop0_lite. case 2 : With this patch (not stopping tick, but explicitly queuing a timer) ============================================================ sample min max 99percentile ============================================================ 20 144us 192us 144us ============================================================ Description of current implementation ------------------------------------- We calculate timeout for the current idle state as the residency value of the next available idle state. If the decrementer is set to be greater than this timeout, we update the decrementer value with the residency of next available idle state. Thus, essentially training the governor to select the next available deeper state until we reach the deepest state. Hence, we won't get stuck unnecessarily in shallow states for longer duration. -------------------------------- v1 of auto-promotion : https://lkml.org/lkml/2019/3/22/58 This patch was implemented only for shallow lite state in generic cpuidle driver. v2 : Removed timeout_needed and rebased to current upstream kernel Then, v1 of forced-wakeup : Moved the code to cpuidle powernv driver and started as forced wakeup instead of auto-promotion v2 : Extended the forced wakeup logic for all states. Setting the decrementer instead of queuing up a hrtimer to implement the logic. v3 : 1) Cleanly handle setting the decrementer after exiting out of stop states. 2) Added a disable_callback feature to compute timeout whenever a state is enbaled or disabled instead of computing everytime in fast idle path. 3) Use disable callback to recompute timeout whenever state usage is changed for a state. Also, cleaned up the get_snooze_timeout function. v4 : Changed the type and name of set/reset decrementer function. Handled irq work pending in try_set_dec_before_idle. No change in patch 2 and 3. Abhishek Goel (3): cpuidle-powernv : forced wakeup for stop states cpuidle : Add callback whenever a state usage is enabled/disabled cpuidle-powernv : Recompute the idle-state timeouts when state usage is enabled/disabled arch/powerpc/include/asm/time.h | 2 ++ arch/powerpc/kernel/time.c | 43 ++++++++++++++++++++++++ drivers/cpuidle/cpuidle-powernv.c | 55 +++++++++++++++++++++++-------- drivers/cpuidle/sysfs.c | 15 ++++++++- include/linux/cpuidle.h | 5 +++ 5 files changed, 106 insertions(+), 14 deletions(-) -- 2.17.1