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SubjectRe: [PATCH v2] watchdog: Fix possible soft lockup warning at bootup
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On 1/16/20 6:44 AM, Thomas Gleixner wrote:
> Thomas Gleixner <tglx@linutronix.de> writes:
>
> Added ARM64 and ThunderX folks
>
>> Waiman Long <longman@redhat.com> writes:
>>> By adding some instrumentation code, it was found that for cpu 14,
>>> watchdog_enable() was called early with a timestamp of 1. That activates
>>> the watchdog time checking logic. It was also found that the monotonic
>>> time measured during the smp_init() phase runs much slower than the
>>> real elapsed time as shown by the below debug printf output:
>>>
>>> [ 1.138522] run_queues, watchdog_timer_fn: now = 170000000
>>> [ 25.519391] run_queues, watchdog_timer_fn: now = 4170000000
>>>
>>> In this particular case, it took about 24.4s of elapsed time for the
>>> clock to advance 4s which is the soft expiration time that is required
>>> to trigger the calling of watchdog_timer_fn(). That clock slowdown
>>> stopped once the smp_init() call was done and the clock time ran at
>>> the same rate as the elapsed time afterward.
> And looking at this with a more awake brain, the root cause is pretty
> obvious.
>
> sched_clock() advances by 24 seconds, but clock MONOTONIC on which the
> watchdog timer is based does not. As the timestamps you printed have 7
> trailing zeros, it's pretty clear that timekeeping is still jiffies
> based at this point and HZ is set to 100.
>
> So while bringing up the non-boot CPUs the boot CPU loses ~2000 timer
> interrupts. That needs to be fixed and not papered over.
>
You are right that the root-causing effort wasn't complete and I took
the easy way out. As I have limited knowledge on how the timer code
work, I was not sure how to start further investigation at that time.
Your insight gives me a hint on where to start now. So I will dig
further in to see what causes this.

Thanks,
Longman

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