linux-kernel - Re: [PATCH] printk/tracing: Do not trace printk_nmi

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Message-ID: <20180907140234.GG24106@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Sep 2018 16:02:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] printk/tracing: Do not trace printk_nmi_enter()

On Fri, Sep 07, 2018 at 09:55:33AM -0400, Steven Rostedt wrote:
> On Fri, 7 Sep 2018 15:45:32 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Yes really, we should not muck with the IRQ state from NMI context.
> 
> Right, and we didn't. Your patch didn't change anything, but allow for

It does, it kills lockdep before we get to mucking with the IRQ state.

> printk_nmi_enter/exit() to be traced by ftrace, but that's wrong to
> begin with because it ftrace_nmi_enter() hasn't been called yet.

That's a second bug :-) A better solution might be to inline that
function as well. A direct OR instruction is still way faster than a
CALL+RET.