linux-kernel - Re: [patch 10/41] x86/apic: Remove the duplicated tracing versions of interrupts

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Date:   Mon, 28 Aug 2017 09:50:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [patch 10/41] x86/apic: Remove the duplicated tracing versions
 of interrupts

On Fri, Aug 25, 2017 at 11:49:47AM -0400, Steven Rostedt wrote:
> On Fri, 25 Aug 2017 12:31:13 +0200
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > The error and the spurious interrupt are really rare events and not at all
> > so performance sensitive that two NOP5s can not be tolerated when tracing
> > is disabled.
> 
> Just a note. I'm sure if we disassembled it, it may be a little more
> work done than just two NOPs, as parameter passing to the tracepoints
> sometimes leak out of the static jump block. It's moot on this patch,
> but other irqs with fast paths may need to be looked at.

Is that something we can fix with the trace macros?

They have a general shape of:

#define trace_foo(args...)
	if (static_branch_unlikely(&foo_enabled)) {
		__trace_foo(args...);
	}

Right? And I suppose I see why the compiler would want to sometimes lift
stuff out of the branch block, but we'd really like it not to do that.
Would putting a barrier() in front of __trace_foo() help?