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? Thanks -- 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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