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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 16 May 2020 22:36:57 -0700
Message-ID: <CALCETrW2negGYRSQ8dqFVdiWGtYpYf6nKe5_hkotNv0e5mNBxA@mail.gmail.com>
Subject: Re: [patch V6 04/37] x86: Make hardware latency tracing explicit
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
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        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Wei Liu <wei.liu@kernel.org>,
        Michael Kelley <mikelley@microsoft.com>,
        Jason Chen CJ <jason.cj.chen@intel.com>,
        Zhao Yakui <yakui.zhao@intel.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
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On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
>
> The hardware latency tracer calls into trace_sched_clock and ends up in
> various instrumentable functions which is problemeatic vs. the kprobe
> handling especially the text poke machinery. It's invoked from
> nmi_enter/exit(), i.e. non-instrumentable code.
>
> Use nmi_enter/exit_notrace() instead. These variants do not invoke the
> hardware latency tracer which avoids chasing down complex callchains to
> make them non-instrumentable.
>
> The real interesting measurement is the actual NMI handler. Add an explicit
> invocation for the hardware latency tracer to it.
>
> #DB and #BP are uninteresting as they really should not be in use when
> analzying hardware induced latencies.
>

> @@ -849,7 +851,7 @@ static void noinstr handle_debug(struct
>  static __always_inline void exc_debug_kernel(struct pt_regs *regs,
>                                              unsigned long dr6)
>  {
> -       nmi_enter();
> +       nmi_enter_notrace();

Why can't exc_debug_kernel() handle instrumentation?  We shouldn't
recurse into #DB since we've already cleared DR7, right?