Currently, the cpuidle governors (menu/ladder) 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 lite stop state, as such lite states will inhibit SMT folding, thereby depriving the other threads in the core from using the core resources. To address this, such lite states need to be autopromoted. The cpuidle-core can queue timer to correspond with the residency value of the next available state. Thus leading to auto-promotion to a deeper idle state as soon as possible. Experiment ---------- Without this patch - It was seen that for a idle system, a cpu may remain in stop0_lite for few seconds and then directly goes to a deeper state such as stop2. With this patch - A cpu will not remain in stop0_lite for more than the residency of next available state, and thus it will go to a deeper state in conservative fashion. Using this, we may spent even less than 20 milliseconds if susbsequent stop states are enabled. In the worst case, we may end up spending more than a second, as was the case without this patch. The worst case will occur in the scenario when no other shallow states are enbaled, and only deep states are available for auto-promotion. Abhishek Goel (2): cpuidle : auto-promotion for cpuidle states cpuidle : Add auto-promotion flag to cpuidle flags arch/powerpc/include/asm/opal-api.h | 1 + drivers/cpuidle/Kconfig | 4 ++++ drivers/cpuidle/cpuidle-powernv.c | 13 +++++++++++-- drivers/cpuidle/cpuidle.c | 3 --- 4 files changed, 16 insertions(+), 5 deletions(-) -- 2.17.1
