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

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Message-ID: <20170522192445.wnkgcxc5d27mt2dz@hirez.programming.kicks-ass.net>
Date:   Mon, 22 May 2017 21:24:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     "mingo@...hat.com" <mingo@...hat.com>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "acme@...hat.com" <acme@...hat.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "vincent.weaver@...ne.edu" <vincent.weaver@...ne.edu>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH 2/2] perf/x86/intel, watchdog: Switch NMI watchdog to ref
 cycles on x86

On Mon, May 22, 2017 at 04:58:04PM +0000, Liang, Kan wrote:
> 
> 
> > On Fri, May 19, 2017 at 10:06:22AM -0700, kan.liang@...el.com wrote:
> > > This patch was once merged, but reverted later.
> > > Because ref-cycles can not be used anymore when watchdog is enabled.
> > > The commit is 44530d588e142a96cf0cd345a7cb8911c4f88720
> > >
> > > The patch 1/2 has extended the ref-cycles to GP counter. The concern
> > > should be gone.
> > 
> > So its not a problem if every Atom prior to Goldmont, and all Core/Core2
> > products regress?
> > 
> > P6 and P4 you've entirely broken, as they don't have REF_CPU_CYCLES at all.
> > 
> > So no, I don't think this is right even now.
> > 
> 
> Right, the patch 1/2 doesn't cover all platforms.
> I will only apply the patch for the platforms,
> which have ref cycles on GP counters.

Right, so if you move the weak function into arch/x86/events/core.c and
simply test for x86_pmu.this_ref_alis_function_thing being !NULL that
should all work.