linux-kernel - Re: [PATCH v1] watchdog: Use a reference cycle counter to avoid scaling issues

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Date:	Fri, 24 Apr 2015 10:44:05 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <ak@...ux.intel.com>
cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Don Zickus <dzickus@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Anton Blanchard <anton@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1] watchdog: Use a reference cycle counter to avoid
 scaling issues

On Thu, 23 Apr 2015, Andi Kleen wrote:
> > We can just detect the deviation in the callback itself:
> > 
> >        u64 now = ktime_get_mono_fast_ns();
> > 
> >        if (now - __this_cpu_read(nmi_timestamp) < period)
> >        	       return;
> > 
> >        __this_cpu_write(nmi_timestamp, now);
> > 
> > It's that simple.
> 
> It's a simple short term hac^wsolution.

Yes, and way simpler and less complex for pushing into stable.

> But if we had a (hypothetical) system with let's say 10*TSC max you
> may end up with quite a few false ticks, as in unnecessary
> interrupts. With 100*TSC it would be really bad.

And hypothetical systems with 100*TSC justify all that?
 
> There were systems in the past that ran TSC at a much slower frequency,
> such as the early AMD Barcelona systems.
> 
> So the problem may eventually come back if not solved properly.

There are better ways to do that than using heuristics. We have to
deal with 3 variants of the reference counter:

1) Core and Atom: counts bus cycles and we know that frequency already
   	    	  from the local apic calibration

2) Nehalem, Westmere: Same as TSC

3) Sandybridge and later:  XCLK which is 100MHz

No magic calibration, just use the information which we have on our
hands already.

Thanks,

	tglx
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