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    SubjectRe: Early timeouts due to inaccurate jiffies during system suspend/resume
    On Mon, Apr 23, 2018 at 08:01:28PM +0300, Imre Deak wrote:
    > On Thu, Apr 19, 2018 at 01:05:39PM +0200, Thomas Gleixner wrote:
    > > On Thu, 19 Apr 2018, Imre Deak wrote:
    > > > Hi,
    > > >
    > > > while checking bug [1], I noticed that jiffies based timing loops like
    > > >
    > > > expire = jiffies + timeout + 1;
    > > > while (!time_after(jiffies, expire))
    > > > do_something;
    > > >
    > > > can last shorter than expected (that is less than timeout).
    > >
    > > Yes, that can happen when the timer interrupt is delayed long enough for
    > > whatever reason. If you need accurate timing then you need to use
    > > ktime_get().
    >
    > Thanks. I always regarded jiffies as non-accurate, but something that
    > gives a minimum time delay guarantee (when adjusted by +1 as above). I
    > wonder if there are other callers in kernel that don't expect an early
    > timeout.

    msleep and any other schedule_timeout based waits are also affected. At the
    same time for example msleep's documentation says:
    "msleep - sleep safely even with waitqueue interruptions".

    To me that suggests a wait with a minimum guaranteed delay.

    Ville had an idea to make the behavior more deterministic by clamping
    the jiffies increment to 1 for each timer interrupt. Would that work?

    >
    > We switched now to using ktime_get_raw() in the i915 driver.
    >
    > >
    > > > After some ftracing it seems like jiffies gets stale due to a missed
    > > > LAPIC timer interrupt after the interrupt is armed in
    > > > lapic_next_deadline() and before jiffies is sampled at 2. above.
    > > > Eventually the interrupt does get delivered, at which point jiffies gets
    > > > updated via tick_do_update_jiffies64() with a >1 ticks increment.
    > > > Between lapic_next_deadline() and the - late - delivery of the interrupt
    > > > the CPU on which the interrupt is armed doesn't go idle.
    > >
    > > That's odd. I have no real explanation for that.
    >
    > Looks like the reason is IRQ latency. For reference here are the
    > longest ones I found with irqsoff ftracing, all running with IRQs disabled
    > during system resume:
    >
    > hpet_rtc_interrupt()->hpet_rtc_timer_reinit():
    > do { ... } while(!hpet_cnt_ahead(...));
    > takes sometimes up to ~40msec for me.
    >
    > hpet_rtc_interrupt()->mc146818_get_time():
    > if (mc146818_is_updating()) mdelay(20);
    >
    > driver_probe_device->atkbd_connect()->i8042_port_close()->__i8042_command()->i8042_wait_write():
    > takes sometimes up to ~10msec for me.
    >
    > All the above paired with asynchronous calling of the drivers' resume
    > hooks may result in the jumps in jiffies I saw.
    >
    > --Imre

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