linux-kernel - Re: [RFC][PATCH 00/14] function_graph: Rewrite to allow multiple users

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Date:   Thu, 22 Nov 2018 14:42:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andy Lutomirski <luto@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [RFC][PATCH 00/14] function_graph: Rewrite to allow multiple
 users

On Thu, Nov 22, 2018 at 07:46:05AM -0500, Steven Rostedt wrote:
> On Thu, 22 Nov 2018 11:08:12 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, Nov 21, 2018 at 08:27:08PM -0500, Steven Rostedt wrote:
> > > Well the fuction graph tracer is arguably the strongest of the tracers.
> > > It shows both the entrance and exit of a function, can give the timings
> > > of a function, and shows the execution of the code quite nicely.
> > > 
> > > But it has one major flaw.
> > > 
> > > It can't let more than one user access it at a time.  
> > 
> > The reason I 'never' use it is performance, it _sucks_.. I've never ran
> > into the multi-user issue.
> 
> And performance is also something to fix (it has improved lately, you
> probably haven't noticed).

Indeed, I've not recently used it.

> > So while I don't think the rewrite is bad, this argument here is.
> 
> Except that we plan on merging kretprobe with function graph tracing.
> This also solves the issue that Mark Rutland has with ret protection.
> He has a solution for function graph tracing, but not with kretprobes.

Sure. But you didn't make that argument.

> And yes, there's also the case of being able to trace to different
> buffers where you can have a full function graph tracing enabled and
> also trace a subset that you want to have.
> 
> Just because you don't need it, doesn't mean it's not needed by others.

Not saying that; just saying the argument doesn't hold. It is not the
_one_ flaw.