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

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Date:   Fri, 22 Apr 2022 12:20:45 +0200
From:   Milian Wolff <milian.wolff@...b.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>
Cc:     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@...r.kernel.org, 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)

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.

> 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?

Thanks
-- 
Milian Wolff | milian.wolff@...b.com | 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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