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
