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From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
To: Thomas Gleixner <tglx@linutronix.de>, Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        x86@kernel.org, Don Zickus <dzickus@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Anton Blanchard <anton@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [PATCH v1] watchdog: Use a reference cycle counter to avoid scaling issues
In-Reply-To: <alpine.DEB.2.11.1504241134090.13914@nanos>
References: <1429801408-11309-1-git-send-email-alexander.shishkin@linux.intel.com> <alpine.DEB.2.11.1504232153080.13914@nanos> <20150423203236.GJ13605@tassilo.jf.intel.com> <alpine.DEB.2.11.1504232242510.13914@nanos> <20150424005133.GK13605@tassilo.jf.intel.com> <alpine.DEB.2.11.1504240957570.13914@nanos> <alpine.DEB.2.11.1504241134090.13914@nanos>
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Date: Fri, 24 Apr 2015 12:46:06 +0300
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Thomas Gleixner <tglx@linutronix.de> writes:

> On Fri, 24 Apr 2015, Thomas Gleixner wrote:
>> 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.
>
> And aside of that calibration stuff emits a warning on everything
> except intel, arc and metag. Very useful.
>
> This is core code and not intel playground.

This warning is only ever compiled on intel and (since last week) on
powerpc.

But sure, it can be fixed.