On 09/18/2012 03:53 AM, Youquan Song wrote: > The prediction for future is difficult and when the cpuidle governor > prediction > fails and govenor possibly choose the shallower C-state than it should. How > to > quickly notice and find the failure becomes important for power saving. > > cpuidle menu governor has a method to predict the repeat pattern if there are > 8 > C-states residency which are continuous and the same or very close, so it will > predict the next C-states residency will keep same residency time. > > This patchset adds a timer when menu governor choose a non-deepest C-state in > order to wake up quickly from shallow C-state to avoid staying too long at > shallow C-state for prediction failure. The timer is set to a time out value > that is greater than predicted time and if the timer with the value is > triggered > , we can confidently conclude prediction is failure. When prediction > succeeds, CPU is waken up from C-states in predicted time and the timer is > not > triggered and will be cancelled right after CPU waken up. When prediction > fails, > the timer is triggered to wake up CPU from shallow C-states, so menu governor > will quickly notice that prediction fails and then re-evaluates deeper > C-states > possibility. This patchset can improves cpuidle prediction process for both > repeat mode and general mode. > > There are 2 cases will clear show this patchset benefit. > > One case is turbostat utility (tools/power/x86/turbostat) at kernel 3.3 or > early > . turbostat utility will read 10 registers one by one at Sandybridge, so it > will > generate 10 IPIs to wake up idle CPUs. So cpuidle menu governor will predict > it > is repeat mode and there is another IPI wake up idle CPU soon, so it keeps > idle > CPU stay at C1 state even though CPU is totally idle. However, in the > turbostat > , following 10 registers reading is sleep 5 seconds by default, so the idle > CPU > will keep at C1 for a long time though it is idle until break event occurs. > In a idle Sandybridge system, run "./turbostat -v", we will notice that deep > C-state dangles between "70% ~ 99%". After patched the kernel, we will notice > deep C-state stays at >99.98%.
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