linux-kernel - Re: [patch V6 04/37] x86: Make hardware latency tracing explicit

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Message-Id: <CF349A31-C401-4684-BA34-CD3359AF34E7@amacapital.net>
Date:   Wed, 20 May 2020 13:14:07 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <JGross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>
Subject: Re: [patch V6 04/37] x86: Make hardware latency tracing explicit



> On May 20, 2020, at 1:10 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> Peter Zijlstra <peterz@...radead.org> writes:
>>> On Mon, May 18, 2020 at 10:05:56AM +0200, Thomas Gleixner wrote:
>>> Peter Zijlstra <peterz@...radead.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);
>>>>> 
>>>>>    instrumentation_begin();
>>>>> +    ftrace_nmi_handler_enter();
>>>>> 
>>>>>    handled = 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);
>>>>> 
>>>>> out:
>>>>> +    ftrace_nmi_handler_exit();
>>>>>    instrumentation_end();
>>>>> }
>>>> 
>>>> 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?
>>> 
>>> Because you then have to fixup _all_ architectures which use
>>> nmi_enter/exit().
>> 
>> We probably have to anyway. But I can do that later I suppose.
> 
> Second thoughts. For #DB and #INT3 we can just keep nmi_enter(), needs
> just annotation in nmi_enter() around that trace muck.
> 
> 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.
> 
> 

What’s the issue?  The actual meat is mostly in the asm for NMI, and for MCE it’s just the sync-all-the-cores thing. The actual simultaneous NMI-and-MCE case is utterly busted regardless, and I’ve been thinking about how to fix it. It won’t be pretty, but nmi_enter() will have nothing to do with it.