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Date:   Fri, 3 Nov 2017 17:12:03 -0400
From:   Luiz Capitulino <lcapitulino@...hat.com>
To:     fweisbec@...il.com
Cc:     tglx@...utronix.de, mtosatti@...hat.com, williams@...hat.com,
        nicstange@...il.com, linux-kernel@...r.kernel.org
Subject: [nohz_full/apic] multiple timer interrupts a second


[CC'ing lkml this time]

Hi,

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.

On further research I saw that Nicolai tried to fix a very similar
problem last year:

commit 1a9e4c564ab174e53ed86def922804a5ddc63e7d
Author: Nicolai Stange <nicstange@...il.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. (!)

Maybe this issue is still present?

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