Re: [RFC] ARM: sched_clock: update epoch_cyc on resume

From: Kevin Hilman
Date: Fri Oct 19 2012 - 19:58:13 EST

Barry Song <21cnbao@xxxxxxxxx> writes:

> 2012/7/18 Colin Cross <ccross@xxxxxxxxxxx>:
>> Many clocks that are used to provide sched_clock will reset during
>> suspend. If read_sched_clock returns 0 after suspend, sched_clock will
>> appear to jump forward. This patch resets cd.epoch_cyc to the current
>> value of read_sched_clock during resume, which causes sched_clock() just
>> after suspend to return the same value as sched_clock() just before
>> suspend.
>> In addition, during the window where epoch_ns has been updated before
>> suspend, but epoch_cyc has not been updated after suspend, it is unknown
>> whether the clock has reset or not, and sched_clock() could return a
>> bogus value. Add a suspended flag, and return the pre-suspend epoch_ns
>> value during this period.
> Acked-by: Barry Song <21cnbao@xxxxxxxxx>
> this patch should also fix the issue that:
> 1. launch some rt threads, rt threads sleep before suspend
> 2. repeat to suspend/resume
> 3. after resuming, waking up rt threads
> repeat 1-3 again and again, sometimes all rt threads will hang after
> resuming due to wrong sched_clock will make sched_rt think rt_time is
> much more than rt_runtime (default 950ms in 1s). then rt threads will
> lost cpu timeslot to run since the 95% threshold is there.

Re-visiting this in light of a related problem.

I've run into a similar issue where IRQ threads are prevented from
running during resume becase the RT throttling kicks because RT
runtime is accumulated during suspend. Using the 'needs_suspend'
version fixes this problem too.

However, because of the RT throttling issue, it seems like *all*
platforms should be using the 'needs_suspend' version always. But, as
already pointed out, that makes the timed printk output during
suspend/resume rather unhelpful.

Having to choose between useful printk times during suspend/resume and
functioning IRQ threads during suspend/resume isn't a choice I want to
make. I'd rather have both. Any ideas?

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