Re: tip.today - scheduler bam boom crash (cpu hotplug)

[Paolo Bonzini]

----- Original Message -----
> From: "Thomas Gleixner" <tglx@xxxxxxxxxxxxx>
> To: "Paolo Bonzini" <pbonzini@xxxxxxxxxx>
> Cc: "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>, "Wanpeng Li" <kernellwp@xxxxxxxxx>, "Mike Galbraith" <efault@xxxxxx>,
> "LKML" <linux-kernel@xxxxxxxxxxxxxxx>, "Ingo Molnar" <mingo@xxxxxxx>, "Borislav Petkov" <bp@xxxxxxxxx>
> Sent: Monday, February 27, 2017 6:40:46 PM
> Subject: Re: tip.today - scheduler bam boom crash (cpu hotplug)
> 
> On Mon, 27 Feb 2017, Paolo Bonzini wrote:
> > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > index 2724dc82f992..3080b6877190 100644
> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -1398,6 +1398,9 @@ void __init tsc_init(void)
> > 
> >  	use_tsc_delay();
> > 
> > +	if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC))
> > +		mark_tsc_unstable("not invariant");
> 
> Errm, no.
> 
> That makes TSC unusable for systems which do not go into C/P states in
> which the TSC stops. There is a world outside KVM ....

Actually I was surprised too by Peter's patch, as it was adding
mark_tsc_unstable pretty much everywhere that didn't have nonstop TSC.
But hopefully it would still be okay to call clear_sched_clock_stable
in tsc_init, in the same way.

Paolo