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SubjectRe: frequent lockups in 3.18rc4
On Sun, 21 Dec 2014, Linus Torvalds wrote:
> On Sun, Dec 21, 2014 at 2:32 PM, Dave Jones <davej@codemonkey.org.uk> wrote:
> > On Sun, Dec 21, 2014 at 02:19:03PM -0800, Linus Torvalds wrote:
> > >
> > > And finally, and stupidly, is there any chance that you have anything
> > > accessing /dev/hpet?
> >
> > Not knowingly at least, but who the hell knows what systemd has its
> > fingers in these days.
>
> Actually, it looks like /dev/hpet doesn't allow write access.
>
> I can do the mmap(/dev/mem) thing and access the HPET by hand, and
> when I write zero to it I immediately get something like this:
>
> Clocksource tsc unstable (delta = -284317725450 ns)
> Switched to clocksource hpet
>
> just to confirm that yes, a jump in the HPET counter would indeed give
> those kinds of symptoms:blaming the TSC with a negative delta in the
> 0-300s range, even though it's the HPET that is broken.
>
> And if the HPET then occasionally jumps around afterwards, it would
> show up as ktime_get() occasionally going backwards, which in turn
> would - as far as I can tell - result in exactly that pseudo-infirnite
> loop with timers.
>
> Anyway, any wild kernel pointer access *could* happen to just hit the
> HPET and write to the main counter value, although I'd personally be
> more inclined to blame BIOS/SMM kind of code playing tricks with
> time.. We do have a few places where we explicitly write the value on
> purpose, but they are in the HPET init code, and in the clocksource
> resume code, so they should not be involved.

Right.

> Thomas - have you had reports of HPET breakage in RT circles, the same
> way BIOSes have been tinkering with TSC?

Not that I'm aware of.

> Also, would it perhaps be a good idea to make "ktime_get()" save the
> last time in a percpu variable, and warn if time ever goes backwards
> on a particular CPU? A percpu thing should be pretty cheap, even if
> we write to it every time somebody asks for time..

That should be simple enough to implement.

Thanks,

tglx


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