Re: [PATCH RESEND 1/1] x86: tsc: avoid system instability in hibernation

[Thomas Gleixner]

On Mon, 30 Jul 2018, Eduardo Valentin wrote:
> On Mon, Jul 30, 2018 at 10:53:54AM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 26, 2018 at 08:56:56AM -0700, Eduardo Valentin wrote:
> > > System instability are seen during resume from hibernation when system
> > > is under heavy CPU load. This is due to the lack of update of sched
> > > clock data
> > 
> > Which would suggest you're already running with unstable sched clock.
> > Otherwise nobody would care about the scd stuff.
> 
> Yes.

I doubt that...

> > 
> > What kind of machine are you running? What does:
> > 
> >   dmesg | grep -i tsc
> > 
> > say?
> 
> Here:
> [    0.000000] tsc: Fast TSC calibration using PIT
> [    0.004005] tsc: Detected 3000.000 MHz processor
> [    0.066796] TSC deadline timer enabled
> [    3.904269] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x2b3e459bf4c, max_idle_ns: 440795289890 ns
>

... because if the sched clock would be unstable then you'd have something
like 'TSC unstable' in dmesg, which you obviously do not.

'sched_clock: Marking unstable' is the other message which would be
emitted.

> > > The fix for this situation is to mark the sched clock as unstable
> > > as early as possible in the resume path, leaving it unstable
> > > for the duration of the resume process. This will force the
> > > scheduler to attempt to align the sched clock across CPUs using
> > > the delta with time of day, updating sched clock data. In a post
> > > hibernation event, we can then mark the sched clock as stable
> > > again, avoiding unnecessary syncs with time of day on systems
> > > in which TSC is reliable.
> > 
> > None of this makes any sense. Either you were already unstable and it
> > should already have worked and them marking it stable is an outright
> > bug, or your sched clock was stable but then your initial diagnosis of
> > lack of scd updates is complete garbage.
> > 
> 
> I see, or it is just a workaround for the underling issue. I, for sure, see no
> lockups anymore after forcing the scd updates. The other thing which are not
> super clear is that this happens during the unfreezing of tasks. If I get a
> set of cpu hog tasks while unfreezing, I see the system throwing worqueue lockup
> detectors in hibernation restore.

Yes, it pretty much papers over something else. Can you please provide a
full dmesg from boot to failure case?

Another question: Does the system recover after issuing the lockup messages
or is it hosed completely?

Thanks,

	tglx