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

[Namhyung Kim]

On Mon, Apr 25, 2022 at 5:42 AM Milian Wolff <milian.wolff@xxxxxxxx> wrote:
>
> 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.

Good point, will add it .

>
> > > > 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?

In my work environment, everything is built with the frame pointer.
It's unfortunate most distros build without it, but as Ian said, I hope
we can lift the limitation with recent technologies soon.

Thanks,
Namhyung