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

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Message-ID: <20150424005133.GK13605@tassilo.jf.intel.com>
Date:	Thu, 23 Apr 2015 17:51:33 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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

> 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. 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.

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.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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