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

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Date:   Fri, 9 Nov 2018 11:05:51 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Steven Rostedt <rostedt@...dmis.org>
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 Nov 9, 2018, at 10:42 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> On Fri, 9 Nov 2018 10:41:37 -0600
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
>>> On Fri, Nov 09, 2018 at 09:21:39AM -0600, Josh Poimboeuf wrote:
>>>> On Fri, Nov 09, 2018 at 07:16:17AM -0800, Andy Lutomirski wrote:  
>>>>> On Thu, Nov 8, 2018 at 11:28 PM Ingo Molnar <mingo@...nel.org> wrote:  
>>>>> 
>>>>> 
>>>>> All other usecases are bonus, but it would certainly be interesting to
>>>>> investigate the impact of using these APIs for tracing: that too is a
>>>>> feature enabled everywhere but utilized only by a small fraction of Linux
>>>>> users - so literally every single cycle or instruction saved or hot-path
>>>>> shortened is a major win.  
>>>> 
>>>> For tracing, we'd want static_call_set_to_nop() or something like that, right?  
>>> 
>>> Are we talking about tracepoints?  Or ftrace?  
>> 
>> Since ftrace changes calls to nops, and vice versa, I assume you meant
>> ftrace.  I don't think ftrace is a good candidate for this, as it's
>> inherently more flexible than this API would reasonably allow.
>> 
> 
> 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.