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Date: Tue, 1 Mar 2016 19:14:04 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>, "Liang, Kan" <kan.liang@intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Stephane Eranian <eranian@google.com>,
        Wang Nan <wangnan0@huawei.com>,
        "zheng.z.yan@intel.com" <zheng.z.yan@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] Core2 cpu triggers hard lockup with perf test
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On Tue, Mar 01, 2016 at 07:04:40PM +0100, Jiri Olsa wrote:

> > That's the PERF_GLOBAL_CTRL, right? But it must have succeeded,
> 
> yep, should be this one:
> 
> static void __intel_pmu_enable_all(int added, bool pmi)
> {
>         struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> 
>         intel_pmu_pebs_enable_all();
>         intel_pmu_lbr_enable_all(pmi);
>  >>>    wrmsrl(MSR_CORE_PERF_GLOBAL_CTRL,
>                         x86_pmu.intel_ctrl & ~cpuc->intel_ctrl_guest_mask);
> 
> 
> > otherwise the NMI watchdog would never have fired.
> 
> so NMI wouldn't trigger if CPU is inside wrmsr?

Well, anything goes with MSR writes, that's all a magic heap of
micro-code.

But at the very least it did actually enable the counters, otherwise the
counter used for the NMI watchdog could not fire, it too would still be
disabled.