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Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup

From John Stultz <john.stultz@linaro.org>
Newsgroups linux.kernel
Subject Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup
Date 2016-01-20 19:00 +0100
Message-ID <qT5d0-1vr-1@gated-at.bofh.it> (permalink)
References (3 earlier) <qSXft-4qR-17@gated-at.bofh.it> <qT4gW-PQ-19@gated-at.bofh.it> <qT4Ah-1fm-13@gated-at.bofh.it> <qT4TE-1oo-21@gated-at.bofh.it> <qT53j-1s5-7@gated-at.bofh.it>
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On Wed, Jan 20, 2016 at 9:42 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Wed, 20 Jan 2016, John Stultz wrote:
>> Ehrm.  A more productive route in solving this might be to cap the
>> cycle delta we return from timekeeping_get_delta().
>>
>> We already do this in the CONFIG_DEBUG_TIMEKEEPING, but adding a
>> simple check it to the non-debug case should be doable w/o adding too
>> much overhead to this very hot path.
>>
>> Something like:
>> if (delta > tkr->clock->max_cycles)
>>     delta = tkr->clock->max_cycles;
>>
>> return delta;
>
> Well, you can make CONFIG_KDB select CONFIG_DEBUG_TIMEKEEPING.

True.  And turning on DEBUG_TIMEKEEPING is probably the easiest thing
for Jeff to try.

Though, there's still the same issue w/ paused VMs. Most of the design
for the timekeeping code has been that it can't properly function if
you block update_wall_time() calls, but it shouldn't kill the box.
With most clocksources, the issue is the counter wraps and we lose
time. But in this case with the TSC its the *very* large cycle delta
turning into a unexpectedly large nanosecond value.

Hrm.. I do also wonder: the logarithmic accumulation chews through
large cycle deltas efficiently, but it does have some design limits,
so it  might also hit the rails and take awhile to spin accumulating
time with such large offsets.

Jeff: Can you try the config option above to let me know if that
avoids the issue? And if not, can you provide some analysis of what
else is going on?

thanks
-john

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Thread

[BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-19 03:00 +0100
  Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-19 03:20 +0100
    Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-19 03:40 +0100
  Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Thomas Gleixner <tglx@linutronix.de> - 2016-01-19 11:00 +0100
  Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-19 16:40 +0100
    Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-19 23:10 +0100
      Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-20 02:00 +0100
      Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Thomas Gleixner <tglx@linutronix.de> - 2016-01-20 10:30 +0100
        Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Thomas Gleixner <tglx@linutronix.de> - 2016-01-20 15:30 +0100
          Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-20 17:50 +0100
        Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-20 18:00 +0100
          Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-20 18:20 +0100
            Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-20 18:40 +0100
            Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup John Stultz <john.stultz@linaro.org> - 2016-01-20 18:40 +0100
              Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Thomas Gleixner <tglx@linutronix.de> - 2016-01-20 18:50 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup John Stultz <john.stultz@linaro.org> - 2016-01-20 19:00 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-20 19:10 +0100
          Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Thomas Gleixner <tglx@linutronix.de> - 2016-01-20 18:30 +0100
            Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-20 18:40 +0100
              Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Thomas Gleixner <tglx@linutronix.de> - 2016-01-20 20:40 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-20 21:10 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-21 08:20 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-21 09:20 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-21 10:10 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Thomas Gleixner <tglx@linutronix.de> - 2016-01-21 11:20 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-21 16:50 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-21 18:00 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-21 18:10 +0100
                Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup Jeff Merkey <linux.mdb@gmail.com> - 2016-01-21 18:10 +0100

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