Re: [RFC] Dynamic Tick and Deferrable Timer Support

From: Jon Hunter
Date: Fri Jan 30 2009 - 14:04:47 EST

john stultz wrote:
As an aside, there are some further hardware limitations in the
timekeeping core that limit the amount of time the hardware can sleep.
For instance, the acpi_pm clocksource wraps every 2.5 seconds or so,
so we have to wake up periodically to sample it to avoid wrapping

Just to be able to deal with all the different hardware out there, the
timekeeping core expects to wake up twice a second to do this
sampling. It may be possible to push this out if you are using other
clocksources (HPET/TSC), but if sleeps for longer then a second are a
needed feature, we probably will need some infrastructure in the
timekeeping core that can be queried to make sure its safe.

The variable "max_delta_ns" is used by the dynamic tick to govern the maximum time a given device can sleep. Hence, this variable should be configured as necessary for the device you are using. Therefore, if your device has a timer that will wrap every 2.5 seconds, then for this device the "max_delta_ns" should be configure so that it does not exceed this time.

So what I was proposing is that for devices that have timers that would allow you to sleep beyond ~2.15 seconds (current max imposed by the clockevent_delta2ns function), why not increase the dynamic range (make this a 64-bit variable) or base (ie. from nanoseconds to milliseconds) to permit longer sleep times for devices that can support them? This should not have any negative impact on devices that cannot support such long sleep times.

So far I have not encountered any issues with doing this. Let me know if this does or does not address your concerns.


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