Messages in this thread | | | Date | Tue, 24 Apr 2018 17:07:41 +0300 | From | Imre Deak <> | Subject | Re: Early timeouts due to inaccurate jiffies during system suspend/resume |
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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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