Messages in this thread | | | Date | Tue, 24 Apr 2018 17:21:01 +0300 | From | Ville Syrjälä <> | Subject | Re: Early timeouts due to inaccurate jiffies during system suspend/resume |
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On Tue, Apr 24, 2018 at 05:07:41PM +0300, Imre Deak wrote: > 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?
Another observation is that this is basically the same problem that we had with the drm vblank counter. I solved that by introducing drm_accurate_vblank_count() which makes sure the counter is up to date before sampling it. Then we can safely do stuff like:
count = drm_accurate_vblank_count(); while (drm_vblank_count() == count) ...;
As long as we don't lose all vblank interrupts that will work and never complete prematurely. And we still allow the vblank counter to increment by >1.
I suppose doing something similar for jiffies would be nice as well, but I'm not sure how feasible that would be. At the very least it would involve patching a lot of code.
> > > > > 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
-- Ville Syrjälä Intel
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