linux-kernel - Re: [RFC PATCH -v2.1] x86: Kill notsc

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Date:	Tue, 17 Nov 2015 11:08:19 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Borislav Petkov <bp@...en8.de>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>, x86-ml <x86@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH -v2.1] x86: Kill notsc

On Tue, 17 Nov 2015, Borislav Petkov wrote:
> On Tue, Nov 17, 2015 at 10:11:03AM +0100, Thomas Gleixner wrote:
> > There is an interesting problem:
> > 
> > tsc_init()
> > {
> >        tsc_khz = x86_platform.calibrate_tsc();
> >        if (!tsc_khz) {
> >        	  mark_tsc_unstable("could not calculate TSC khz");
> > 	  ...
> >        }
> > 
> > In the current code we do NOT use TSC for sched_clock() and that's
> > correct as we have no idea what the TSC frequency is.
> > 
> > With your changes that is not longer the case, so you end up with a
> > completely wreckaged sched clock.
> 
> Hmm, I see it.
> 
> Well, I had a related splat which bombed because cyc2ns_init() had to
> run before we do sched_init()->idle_init() which called sched_clock()
> and there we did the cycles_2_ns() thing and the percpu vars weren't
> initialized yet.
> 
> That's why I did this:
> 
> +void __init early_tsc_init(void)
> +{
> +       int cpu;
> +
> +       /*
> +        * We need to init the cycles to ns conversion machinery because
> +        * init_idle() below will call sched_clock() which needs it.
> +        */
> +       for_each_possible_cpu(cpu)
> +               cyc2ns_init(cpu);
> +}
> 
> Maybe I should move tsc_init() before sched_init() so that the
> calibration happens before we use the TSC in sched_clock() for the first
> time...?

How does that help if your TSC frequency is 0 after the calibration?

Thanks,

	tglx
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