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

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Message-ID: <87imgqv055.fsf@nanos.tec.linutronix.de>
Date:   Wed, 20 May 2020 22:09:42 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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

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.

Thanks,

        tglx