linux-kernel - Re: [PATCH v9 00/36] tracing: fprobe: function_graph: Multi-function graph and fprobe on fgraph

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Date: Mon, 29 Apr 2024 22:51:19 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Steven Rostedt
 <rostedt@...dmis.org>, Florent Revest <revest@...omium.org>,
 linux-trace-kernel@...r.kernel.org, LKML <linux-kernel@...r.kernel.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>,
 Mark Rutland <mark.rutland@....com>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH v9 00/36] tracing: fprobe: function_graph:
 Multi-function graph and fprobe on fgraph

Hi Andrii,

On Thu, 25 Apr 2024 13:31:53 -0700
Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:

> Hey Masami,
> 
> I can't really review most of that code as I'm completely unfamiliar
> with all those inner workings of fprobe/ftrace/function_graph. I left
> a few comments where there were somewhat more obvious BPF-related
> pieces.
> 
> But I also did run our BPF benchmarks on probes/for-next as a baseline
> and then with your series applied on top. Just to see if there are any
> regressions. I think it will be a useful data point for you.

Thanks for testing!

> 
> You should be already familiar with the bench tool we have in BPF
> selftests (I used it on some other patches for your tree).

What patches we need?

> 
> BASELINE
> ========
> kprobe         :   24.634 ± 0.205M/s
> kprobe-multi   :   28.898 ± 0.531M/s
> kretprobe      :   10.478 ± 0.015M/s
> kretprobe-multi:   11.012 ± 0.063M/s
> 
> THIS PATCH SET ON TOP
> =====================
> kprobe         :   25.144 ± 0.027M/s (+2%)
> kprobe-multi   :   28.909 ± 0.074M/s
> kretprobe      :    9.482 ± 0.008M/s (-9.5%)
> kretprobe-multi:   13.688 ± 0.027M/s (+24%)

This looks good. Kretprobe should also use kretprobe-multi (fprobe)
eventually because it should be a single callback version of
kretprobe-multi.

> 
> These numbers are pretty stable and look to be more or less representative.
> 
> As you can see, kprobes got a bit faster, kprobe-multi seems to be
> about the same, though.
> 
> Then (I suppose they are "legacy") kretprobes got quite noticeably
> slower, almost by 10%. Not sure why, but looks real after re-running
> benchmarks a bunch of times and getting stable results.

Hmm, kretprobe on x86 should use ftrace + rethook even with my series.
So nothing should be changed. Maybe cache access pattern has been
changed?
I'll check it with tracefs (to remove the effect from bpf related changes)

> 
> On the other hand, multi-kretprobes got significantly faster (+24%!).
> Again, I don't know if it is expected or not, but it's a nice
> improvement.

Thanks!

> 
> If you have any idea why kretprobes would get so much slower, it would
> be nice to look into that and see if you can mitigate the regression
> somehow. Thanks!

OK, let me check it.

Thank you!

> 
> 
> >  51 files changed, 2325 insertions(+), 882 deletions(-)
> >  create mode 100644 tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe_repeat.tc
> >
> > --
> > Masami Hiramatsu (Google) <mhiramat@...nel.org>
> >


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>