linux-perf-users.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Milian Wolff <milian.wolff@kdab.com>
To: Namhyung Kim <namhyung@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andi Kleen <ak@linux.intel.com>, Ian Rogers <irogers@google.com>,
	Song Liu <songliubraving@fb.com>, Hao Luo <haoluo@google.com>,
	bpf <bpf@vger.kernel.org>,
	linux-perf-users <linux-perf-users@vger.kernel.org>,
	Blake Jones <blakejones@google.com>
Subject: Re: [RFC 0/4] perf record: Implement off-cpu profiling with BPF (v1)
Date: Mon, 25 Apr 2022 14:42:27 +0200	[thread overview]
Message-ID: <5616892.dGzqbEiDyy@milian-workstation> (raw)
In-Reply-To: <CAM9d7cjU6RGMStG4YOW5D50Sx4PRia2dfzcPKxb4JLh5Q668mw@mail.gmail.com>

[-- Attachment #1: Type: text/plain, Size: 3142 bytes --]

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@kdab.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.

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?

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

[-- Attachment #2: smime.p7s --]
[-- Type: application/pkcs7-signature, Size: 5272 bytes --]

  parent reply	other threads:[~2022-04-25 12:44 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-22  5:33 [RFC 0/4] perf record: Implement off-cpu profiling with BPF (v1) Namhyung Kim
2022-04-22  5:33 ` [PATCH 1/4] perf report: Do not extend sample type of bpf-output event Namhyung Kim
2022-04-22  5:33 ` [PATCH 2/4] perf record: Enable off-cpu analysis with BPF Namhyung Kim
2022-04-22  5:34 ` [PATCH 3/4] perf record: Implement basic filtering for off-cpu Namhyung Kim
2022-04-22  5:34 ` [PATCH 4/4] perf record: Handle argument change in sched_switch Namhyung Kim
2022-04-22 10:11 ` [RFC 0/4] perf record: Implement off-cpu profiling with BPF (v1) Jiri Olsa
2022-04-22 14:53   ` Namhyung Kim
2022-04-22 10:20 ` Milian Wolff
2022-04-22 15:01   ` Namhyung Kim
2022-04-22 19:04     ` Arnaldo Carvalho de Melo
2022-04-25 12:42     ` Milian Wolff [this message]
2022-04-25 16:49       ` Ian Rogers
2022-04-25 18:58       ` Namhyung Kim

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=5616892.dGzqbEiDyy@milian-workstation \
    --to=milian.wolff@kdab.com \
    --cc=acme@kernel.org \
    --cc=ak@linux.intel.com \
    --cc=blakejones@google.com \
    --cc=bpf@vger.kernel.org \
    --cc=haoluo@google.com \
    --cc=irogers@google.com \
    --cc=jolsa@kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-perf-users@vger.kernel.org \
    --cc=mingo@kernel.org \
    --cc=namhyung@kernel.org \
    --cc=peterz@infradead.org \
    --cc=songliubraving@fb.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).