Re: [PATCH] PM / Domains: Skip latency measurements if timekeeping is suspended

On Wednesday, March 18, 2015 05:25:46 PM Geert Uytterhoeven wrote:

The PM Domain code uses ktime_get() to perform various latency
measurements.  However, if ktime_get() is called while timekeeping is
suspended, the following warning is printed:

    WARNING: CPU: 0 PID: 1340 at kernel/time/timekeeping.c:576 ktime_get+0x30/0xf4()

This happens when resuming the PM Domain that contains the clock events
source. Chain of operations is:

    timekeeping_resume()
    {
        clockevents_resume()
            sh_cmt_clock_event_resume()
                pm_genpd_syscore_poweron()
                    pm_genpd_sync_poweron()
                        genpd_power_on()
                            ktime_get(), but timekeeping_suspended == 1
        ...
        timekeeping_suspended = 0;
    }

Skip all latency measurements if timekeeping is suspended to fix this.

I don't think that this is where we should fix it.  At least using
timekeeping_suspended outside of the timekeeping core would not be
welcome by its maintainers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
I'm not sure if this is needed for all latency measurements.
So far I only encountered it while powering-on a clock domain during
resume from s2ram.

The problem seems to be that the clock domain is powered on in a
syscore resume routine which happens to be called before timekeeping_resume().
It looks like we either need to force the right ordering somehow or have a
special variant of GENPD_DEV_TIMED_CALLBACK() for syscore suspend/resume that
won't do the latency measurement at all (which doesn't make much sense at
this point, because time is effectively "frozen" then).

Rafael