Messages in this thread | | | Date | Mon, 23 Apr 2018 20:01:28 +0300 | From | Imre Deak <> | Subject | Re: Early timeouts due to inaccurate jiffies during system suspend/resume |
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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.
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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