Re: [nohz_full/apic] multiple timer interrupts a second

[Nicolai Stange]

Hi Luiz,

[John Stultz added to CC]

On Fri, Nov 03 2017, Luiz Capitulino wrote:

> [CC'ing lkml this time]
>
> I've observed that smp_apic_timer_interrupt() is sometimes called
> two or more times a second on a nohz_full core which has a single
> task taking 100% of the core. In one of the calls, hrtimer_interrupt()
> runs tick_sched_timer(), but in others it doesn't call any handler.
> Here's an example (Linus HEAD f34157878):
>
> <...>-1831  [008]  1060.115578: funcgraph_entry:                   |  smp_apic_timer_interrupt() {
> <...>-1831  [008]  1060.115578: funcgraph_entry:                   |    hrtimer_interrupt() {
> <...>-1831  [008]  1060.115578: hrtimer_expire_entry: hrtimer=0xffff8edfefd12d60 function=tick_sched_timer now=1060079001980
> <...>-1831  [008]  1060.115578: funcgraph_entry:        1.172 us   |      tick_sched_timer();
> <...>-1831  [008]  1060.115580: funcgraph_exit:         1.757 us   |    }
> <...>-1831  [008]  1060.115580: hrtimer_start:        hrtimer=0xffff8edfefd12d60 function=tick_sched_timer expires=1061079000000 softexpires=1061079000000
> <...>-1831  [008]  1060.115581: funcgraph_exit:         3.026 us   |  }
> <...>-1831  [008]  1061.115577: funcgraph_entry:                   |  smp_apic_timer_interrupt() {
> <...>-1831  [008]  1061.115577: funcgraph_entry:        0.261 us   |    hrtimer_interrupt(); <---------- NO handler called
> <...>-1831  [008]  1061.115578: funcgraph_exit:         1.349 us   |  }
> <...>-1831  [008]  1061.115579: funcgraph_entry:                   |  smp_apic_timer_interrupt() {
> <...>-1831  [008]  1061.115579: funcgraph_entry:                   |    hrtimer_interrupt() {
> <...>-1831  [008]  1061.115579: hrtimer_expire_entry: hrtimer=0xffff8edfefd12d60 function=tick_sched_timer now=1061079001473
> <...>-1831  [008]  1061.115580: funcgraph_entry:        1.413 us   |      tick_sched_timer();
> <...>-1831  [008]  1061.115581: funcgraph_exit:         2.124 us   |    }
> <...>-1831  [008]  1061.115582: hrtimer_start:        hrtimer=0xffff8edfefd12d60 function=tick_sched_timer expires=1062079000000 softexpires=1062079000000
> <...>-1831  [008]  1061.115582: funcgraph_exit:         3.255 us   |  }
>
> Is this expected for some reason?
>
> I guess what's happening is that the deadline timer is firing
> earlier than expected. From a few dozen to a few hundreds
> nanoseconds earlier. When this happens, hrtimer_interrupt()
> skips calling the hrtimer handler (since it's early) and the
> apic is programmed to fire in the next microsecond.

Exactly.


> On further research I saw that Nicolai tried to fix a very similar
> problem last year:
>
> commit 1a9e4c564ab174e53ed86def922804a5ddc63e7d
> Author: Nicolai Stange <nicstange@xxxxxxxxx>
> Date:   Thu Jul 14 17:22:54 2016 +0200
>
>     x86/timers/apic: Fix imprecise timer interrupts by eliminating TSC clockevents frequency roundoff error
>     
>     I noticed the following bug/misbehavior on certain Intel systems: with a
>     single task running on a NOHZ CPU on an Intel Haswell, I recognized
>     that I did not only get the one expected local_timer APIC interrupt, but
>     two per second at minimum. (!)

Note that there's also 6731b0d611a1 ("x86/timers/apic: Inform TSC
deadline clockevent device about recalibration").


> Maybe this issue is still present?

Yes it is. The reason is that NTP frequency adjustments would make it
into the mono clocksource's frequency but not into the
clock_event_device's.

I sent a series back then [1] addressing this.

However, in the meanwhile, I changed jobs and thus, got somehow
distracted. My bad :/

The result is that this got merged only up to what corresponds to
[10/28] in [1].

I'll pick this up again now and try to get the rest accepted for
inclusion.


Thanks,

Nicolai


[1] last time I sent this series as a whole can be found at
    http://lkml.kernel.org/r/20161119160055.12491-2-nicstange@xxxxxxxxx