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

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Message-ID: <874ksdzmzf.fsf@nanos.tec.linutronix.de>
Date:   Mon, 18 May 2020 10:05:56 +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 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().

Thanks,

        tglx