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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [patch V6 04/37] x86: Make hardware latency tracing explicit
Date:   Wed, 20 May 2020 13:14:07 -0700
Message-Id: <CF349A31-C401-4684-BA34-CD3359AF34E7@amacapital.net>
References: <87imgqv055.fsf@nanos.tec.linutronix.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, 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>,
        Juergen Gross <JGross@suse.com>,
        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>
In-Reply-To: <87imgqv055.fsf@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On May 20, 2020, at 1:10 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
> =EF=BB=BFPeter Zijlstra <peterz@infradead.org> writes:
>>> On Mon, May 18, 2020 at 10:05:56AM +0200, Thomas Gleixner wrote:
>>> Peter Zijlstra <peterz@infradead.org> writes:
>>>> On Sat, May 16, 2020 at 01:45:51AM +0200, Thomas Gleixner wrote:
>>>>> --- a/arch/x86/kernel/nmi.c
>>>>> +++ b/arch/x86/kernel/nmi.c
>>>>> @@ -334,6 +334,7 @@ static noinstr void default_do_nmi(struc
>>>>>    __this_cpu_write(last_nmi_rip, regs->ip);
>>>>>=20
>>>>>    instrumentation_begin();
>>>>> +    ftrace_nmi_handler_enter();
>>>>>=20
>>>>>    handled =3D nmi_handle(NMI_LOCAL, regs);
>>>>>    __this_cpu_add(nmi_stats.normal, handled);
>>>>> @@ -420,6 +421,7 @@ static noinstr void default_do_nmi(struc
>>>>>        unknown_nmi_error(reason, regs);
>>>>>=20
>>>>> out:
>>>>> +    ftrace_nmi_handler_exit();
>>>>>    instrumentation_end();
>>>>> }
>>>>=20
>>>> Yeah, so I'm confused about this and the previous patch too. Why not
>>>> do just this? Remove that ftrace_nmi_handler.* crud from
>>>> nmi_{enter,exit}() and stick it here? Why do we needs the
>>>> nmi_{enter,exit}_notrace() thing?
>>>=20
>>> Because you then have to fixup _all_ architectures which use
>>> nmi_enter/exit().
>>=20
>> We probably have to anyway. But I can do that later I suppose.
>=20
> Second thoughts. For #DB and #INT3 we can just keep nmi_enter(), needs
> just annotation in nmi_enter() around that trace muck.
>=20
> For #NMI and #MCE I rather avoid the early trace call and do it once we
> have reached "stable" state, i.e. avoid it in the whole nested NMI mess.
>=20
>=20

What=E2=80=99s the issue?  The actual meat is mostly in the asm for NMI, and=
 for MCE it=E2=80=99s just the sync-all-the-cores thing. The actual simultan=
eous NMI-and-MCE case is utterly busted regardless, and I=E2=80=99ve been th=
inking about how to fix it. It won=E2=80=99t be pretty, but nmi_enter() will=
 have nothing to do with it.=