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Date:   Mon, 2 Oct 2023 22:52:42 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Liang, Kan" <kan.liang@linux.intel.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Ingo Molnar <mingo@kernel.org>,
        Dapeng Mi <dapeng1.mi@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Like Xu <likexu@tencent.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>, kvm@vger.kernel.org,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Zhang Xiong <xiong.y.zhang@intel.com>,
        Lv Zhiyuan <zhiyuan.lv@intel.com>,
        Yang Weijiang <weijiang.yang@intel.com>,
        Dapeng Mi <dapeng1.mi@intel.com>,
        Jim Mattson <jmattson@google.com>,
        David Dunn <daviddunn@google.com>,
        Mingwei Zhang <mizhang@google.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics
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On Mon, Oct 02, 2023 at 03:50:24PM -0400, Liang, Kan wrote:

> Now, the NMI watchdog is using a "CPU-pinned" event. But I think it can
> be replaced by the buddy system, commit 1f423c905a6b
> ("watchdog/hardlockup: detect hard lockups using secondary (buddy) CPUs")

For some simple cases. I've had plenty experience with situations where
that thing would be completely useless.

That is, at some point the 'all CPUs hard locked up' scenario was
something I ran into a lot (although I can't for the life of me remember
wtf I was doing at the time). All that needs is a single
spin_lock_irqsave() on a common lock (or group of locks, like the
rq->lock). Before you know it, the whole machine is a brick.

That said; if you augment this thing with a bunch of CPUs that have
HPET-NMI and IPI-NMI for backtraces, it might actually be useful.