Re: [RFC 0/4] perf record: Implement off-cpu profiling with BPF (v1)

From: Milian Wolff
Date: Mon Apr 25 2022 - 08:44:56 EST

On Freitag, 22. April 2022 17:01:15 CEST Namhyung Kim wrote:
> Hi Milian,
> On Fri, Apr 22, 2022 at 3:21 AM Milian Wolff <milian.wolff@xxxxxxxx> wrote:
> > On Freitag, 22. April 2022 07:33:57 CEST Namhyung Kim wrote:
> > > Hello,
> > >
> > > This is the first version of off-cpu profiling support. Together with
> > > (PMU-based) cpu profiling, it can show holistic view of the performance
> > > characteristics of your application or system.
> >
> > Hey Namhyung,
> >
> > this is awesome news! In hotspot, I've long done off-cpu profiling
> > manually by looking at the time between --switch-events. The downside is
> > that we also need to track the sched:sched_switch event to get a call
> > stack. But this approach also works with dwarf based unwinding, and also
> > includes kernel stacks.
> Thanks, I've also briefly thought about the switch event based off-cpu
> profiling as it doesn't require root. But collecting call stacks is hard
> and I'd like to do it in kernel/bpf to reduce the overhead.

I'm all for reducing the overhead, I just wonder about the practicality. At
the very least, please make sure to note this limitation explicitly to end
users. As a preacher for perf, I have come across lots of people stumbling
over `perf record -g` not producing any sensible output because they are
simply not aware that this requires frame pointers which are basically non
existing on most "normal" distributions. Nowadays `man perf record` tries to
educate people, please do the same for the new `--off-cpu` switch.

> > > With BPF, it can aggregate scheduling stats for interested tasks
> > > and/or states and convert the data into a form of perf sample records.
> > > I chose the bpf-output event which is a software event supposed to be
> > > consumed by BPF programs and renamed it as "offcpu-time". So it
> > > requires no change on the perf report side except for setting sample
> > > types of bpf-output event.
> > >
> > > Basically it collects userspace callstack for tasks as it's what users
> > > want mostly. Maybe we can add support for the kernel stacks but I'm
> > > afraid that it'd cause more overhead. So the offcpu-time event will
> > > always have callchains regardless of the command line option, and it
> > > enables the children mode in perf report by default.
> >
> > Has anything changed wrt perf/bpf and user applications not compiled with
> > `- fno-omit-frame-pointer`? I.e. does this new utility only work for
> > specially compiled applications, or do we also get backtraces for
> > "normal" binaries that we can install through package managers?
> I am not aware of such changes, it still needs a frame pointer to get
> backtraces.

May I ask what kind of setup you are using this on? Do you use something like
Gentoo or yocto where you compile your whole system with `-fno-omit-frame-
pointer`? Because otherwise, any kind of off-cpu time in system libraries will
not be resolved properly, no?

Milian Wolff | milian.wolff@xxxxxxxx | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts

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