RE: [PATCH v3] cpuidle: poll_state: Add time limit to poll_idle()

[Doug Smythies]

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