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

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Message-ID: <20181122074605.063a7525@vmware.local.home>
Date:   Thu, 22 Nov 2018 07:46:05 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.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, 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).

> 
> 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.

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.

-- Steve