Re: [RFC 0/3] watchdog/softlockup: Make softlockup reports more reliable and useful

[Thomas Gleixner]

On Wed, 5 Jun 2019, Petr Mladek wrote:

> Hi,
> 
> we were analyzing logs with several softlockup reports in flush_tlb_kernel_range().
> They were confusing. Especially it was not clear whether it was deadlock,
> livelock, or separate softlockups.
> 
> It went out that even a simple busy loop:
> 
> 	while (true)
> 	      cpu_relax();
> 
> is able to produce several softlockups reports:
> 
> [  168.277520] watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [cat:4865]
> [  196.277604] watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [cat:4865]
> [  236.277522] watchdog: BUG: soft lockup - CPU#1 stuck for 23s! [cat:4865]
> 
> 
> I tried to understand the tricky watchdog code and produced two patches
> that would be helpful to debug the original real bug:
> 
>    1st patch prevents restart of the watchdog from unrelated locations.
> 
>    2nd patch helps to distinguish several possible situations by
>    regular reports.
> 
>    3rd patch can be used for testing the problem.
> 
> 
> The watchdog code might deserve even more clean up. Anyway, I would
> like to hear other's opinion first.

Anything which improves debugability is welcome. Unfortunately you missed
to add an example of the output after these patches are applied.

Thanks,

	tglx