linux-kernel - Re: [PATCH 2/2] perf/x86/intel, watchdog: Switch NMI watchdog to ref cycles on x86

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Message-ID: <20170522200144.GE15563@tassilo.jf.intel.com>
Date:   Mon, 22 May 2017 13:01:44 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     "Liang, Kan" <kan.liang@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vince Weaver <vincent.weaver@...ne.edu>
Subject: Re: [PATCH 2/2] perf/x86/intel, watchdog: Switch NMI watchdog to ref
 cycles on x86

> 
> > The ref cycles always tick at their frequency, or slower when the system
> > is idling. That means the NMI watchdog can never expire too early,
> > unlike with cycles.
> >
> Just make the period longer, like 30% longer. Take the max turbo factor you can
> get and use that. It is okay if it takes longer of machine with
> smaller max Turbo ratios.

That would be a ticking time bomb. Turbo ratios are likely to grow.

There's no architectural way to get a turbo factor. Even if there
is a model specific MSR it would likely not work in virtualization etc.

You could make the timeout ridiculously large, but even that would
have a small chance of failure.

-Andi