linux-kernel - Re: [PATCH RFC 0/3] Static calls

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Date:   Fri, 9 Nov 2018 14:37:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH RFC 0/3] Static calls

On Fri, 9 Nov 2018 11:05:51 -0800
Andy Lutomirski <luto@...capital.net> wrote:

> > Not sure what Andy was talking about, but I'm currently implementing
> > tracepoints to use this, as tracepoints use indirect calls, and are a
> > prime candidate for static calls, as I showed in my original RFC of
> > this feature.
> > 
> >   
> 
> Indeed.
> 
> Although I had assumed that tracepoints already had appropriate jump label magic.

It does. But that's not the problem I was trying to solve. It's that
tracing took a 8% noise dive with retpolines when enabled (hackbench
slowed down by 8% with all the trace events enabled compared to all
trace events enabled without retpoline). That is, normal users (those
not tracinng) are not affected by trace events slowing down by
retpoline. Those that care about performance when they are tracing, are
affected by retpoline, quite drastically.

I'm doing another test run and measurements, to see how the unoptimized
trampolines help, followed by the trampoline case.

-- Steve