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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Sat, 04 Nov 2017 10:14:52 +0100
Nicolai Stange <nicstange@gmail.com> wrote:

> 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@gmail.com>
> > 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.

OK. If you get a new version, please include me in the CC so that
I can give it a try.

> 
> 
> 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@gmail.com
>