[RFC] ARM: sched_clock: update epoch_cyc on resume

[Barry Song]

2012/7/18 Colin Cross <ccross at android.com>:
> 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 at gmail.com>

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.

>
> This will have a side effect of causing SoCs that have clocks that
> continue to count in suspend to appear to stop counting, reporting the
> same sched_clock() value before and after suspend.
>
> Signed-off-by: Colin Cross <ccross at android.com>
> ---
>  arch/arm/kernel/sched_clock.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
>
> diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c
> index 27d186a..46c7d32 100644
> --- a/arch/arm/kernel/sched_clock.c
> +++ b/arch/arm/kernel/sched_clock.c
> @@ -21,6 +21,7 @@ struct clock_data {
>         u32 epoch_cyc_copy;
>         u32 mult;
>         u32 shift;
> +       bool suspended;
>  };
>
>  static void sched_clock_poll(unsigned long wrap_ticks);
> @@ -49,6 +50,9 @@ static unsigned long long cyc_to_sched_clock(u32 cyc, u32 mask)
>         u64 epoch_ns;
>         u32 epoch_cyc;
>
> +       if (cd.suspended)
> +               return cd.epoch_ns;
> +
>         /*
>          * Load the epoch_cyc and epoch_ns atomically.  We do this by
>          * ensuring that we always write epoch_cyc, epoch_ns and
> @@ -169,11 +173,20 @@ void __init sched_clock_postinit(void)
>  static int sched_clock_suspend(void)
>  {
>         sched_clock_poll(sched_clock_timer.data);
> +       cd.suspended = true;
>         return 0;
>  }
>
> +static void sched_clock_resume(void)
> +{
> +       cd.epoch_cyc = read_sched_clock();
> +       cd.epoch_cyc_copy = cd.epoch_cyc;
> +       cd.suspended = false;
> +}
> +
>  static struct syscore_ops sched_clock_ops = {
>         .suspend = sched_clock_suspend,
> +       .resume = sched_clock_resume,
>  };
>
>  static int __init sched_clock_syscore_init(void)


-barry