linux-kernel - Re: [PATCH v2 24/27] function_graph: Use static_call and branch to optimize entry function

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Message-ID: <20240603110018.1cdd6746@gandalf.local.home>
Date: Mon, 3 Jun 2024 11:00:18 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Mark
 Rutland <mark.rutland@....com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Alexei Starovoitov
 <alexei.starovoitov@...il.com>, Florent Revest <revest@...omium.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, bpf <bpf@...r.kernel.org>, Sven
 Schnelle <svens@...ux.ibm.com>, Alexei Starovoitov <ast@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Arnaldo Carvalho de Melo <acme@...nel.org>, Daniel
 Borkmann <daniel@...earbox.net>, Alan Maguire <alan.maguire@...cle.com>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner
 <tglx@...utronix.de>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH v2 24/27] function_graph: Use static_call and branch to
 optimize entry function

On Mon, 3 Jun 2024 12:11:07 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> > From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> > 
> > In most cases function graph is used by a single user. Instead of calling
> > a loop to call function graph callbacks in this case, call the function
> > entry callback directly.
> > 
> > Add a static_key that will be used to set the function graph logic to
> > either do the loop (when more than one callback is registered) or to call
> > the callback directly if there is only one registered callback.  
> 
> I understand this works, but my concern is that, if we use fprobe
> and function_graph at the same time, does it always loop on both gops?
> 
> I mean if those are the subops of one ftrace_ops, ftrace_trampoline
> will always call the same function_graph_enter() for both gops, and loop
> on the gops list.
> 
> For example, if there are 2 fgraph_ops, one has "vfs_*" filter and
> another has "sched_*" filter, those does not cover each other.
> 
> Are there any way to solve this issue? I think my previous series
> calls function_graph_enter_ops() directly from trampoline (If it works
> correctly...)

Yes, but that gets a bit complex, and requires the changing of all archs.
If it starts to become a problem, I rather add that as a feature. That is,
we can always go back to it. But for now, lets keep the complexity down.

-- Steve