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

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Date:   Fri, 22 Apr 2022 08:01:15 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Milian Wolff <milian.wolff@...b.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Song Liu <songliubraving@...com>, Hao Luo <haoluo@...gle.com>,
        bpf <bpf@...r.kernel.org>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        Blake Jones <blakejones@...gle.com>
Subject: Re: [RFC 0/4] perf record: Implement off-cpu profiling with BPF (v1)

Hi Milian,

On Fri, Apr 22, 2022 at 3:21 AM Milian Wolff <milian.wolff@...b.com> 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.

>
> > 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.

Thanks,
Namhyung