On 2018.03.14 07:04 Rafael J. Wysocki wrote: > If poll_idle() is allowed to spin until need_resched() returns 'true', > it may actually spin for a much longer time than expected by the idle > governor, since set_tsk_need_resched() is not always called by the > timer interrupt handler. If that happens, the CPU may spend much > more time than anticipated in the "polling" state. > > To prevent that from happening, limit the time of the spinning loop > in poll_idle().
...[snip]... > +#define POLL_IDLE_TIME_LIMIT (TICK_NSEC / 16) The other ongoing threads on this aside, potentially, there might be another issue. What if the next available idle state, after 0, has a residency that is greater than TICK_NSEC / 16? Meaning these numbers, for example: /sys/devices/system/cpu/cpu0/cpuidle/state*/residency The suggestion is that upon a timeout exit from idle state 0, the measured_us should maybe be rejected, because the statistics are being biased and it doesn't seem to correct itself. Up to 1300% (<- not a typo) extra power consumption has been observed. Supporting experimental data: My processor: /sys/devices/system/cpu/cpu0/cpuidle/state0/residency:0 /sys/devices/system/cpu/cpu0/cpuidle/state1/residency:2 /sys/devices/system/cpu/cpu0/cpuidle/state2/residency:20 /sys/devices/system/cpu/cpu0/cpuidle/state3/residency:211 <<< Important /sys/devices/system/cpu/cpu0/cpuidle/state4/residency:345 A 1000 Hz kernel (TICK_NSEC/16) = 62.5 nsec; idle system: Idle state 0 time: Typically 0 uSec. Processor package power: 3.7 watts (steady) Now, disable idle states 1 and 2: Idle state 0 time (all 8 CPUs): ~~ 430 Seconds / minute Processor package power: ~52 watts (1300% more power, 14X) A 250 Hz kernel (TICK_NSEC/16) = 250 nSec; idle system: Idle state 0 time: Typically < 1 mSec / minute Processor package power: 3.7 to 3.8 watts Now, disable idle states 1 and 2: Idle state 0 time (all 8 CPUs): Typically 0 to 70 mSecs / minute Processor package power: 3.7 to 3.8 watts A 1000 Hz kernel with: +#define POLL_IDLE_TIME_LIMIT (TICK_NSEC / 4) Note: Just for a test. I am not suggesting this should change. instead. i.e. (TICK_NSEC/4) = 250 nSec. Idle state 0 time: Typically 0 uSec. Processor package power: 3.7 watts (steady) Now, disable idle states 1 and 2: Idle state 0 time (all 8 CPUs): Typically 0 to 70 mSecs / minute Processor package power: ~3.8 watts Note 1: My example is contrived via disabling idle states, so I don't know if it actually needs to be worried about. Note 2: I do not know if there is some processor where cpuidle/state1/residency is > 62.5 nSec. Note 3: I am trying to figure out a way to test rejecting measured_us upon timeout exit, but haven't made much progress. ... Doug
