On Thu, Dec 22, 2022 at 02:25:49PM -0800, Namhyung Kim wrote: > On Thu, Dec 22, 2022 at 12:16 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > On Thu, Dec 22, 2022 at 09:34:42AM -0800, Namhyung Kim wrote: > > > > > Sorry about that. Let me rephrase it like below: > > > > > > With bpf_cast_to_kern_ctx(), BPF programs attached to a perf event > > > can access perf sample data directly from the ctx. > > > > This is the bpf_prog_run() in bpf_overflow_handler(), right? > > Yes. > > > > > > But the perf sample > > > data is not fully prepared at this point, and some fields can have invalid > > > uninitialized values. So it needs to call perf_prepare_sample() before > > > calling the BPF overflow handler. > > > > It never was, why is it a problem now? > > BPF used to allow selected fields only like period and addr, and they > are initialized always by perf_sample_data_init(). This is relaxed > by the bpf_cast_to_kern_ctx() and it can easily access arbitrary > fields of perf_sample_data now. > > The background of this change is to use BPF as a filter for perf > event samples. The code is there already and returning 0 from > BPF can drop perf samples. With access to more sample data, > it'd make more educated decisions. > > For example, I got some requests to limit perf samples in a > selected region of address (code or data). Or it can collect > samples only if some hardware specific information is set in > the raw data like in AMD IBS. We can easily extend it to other > sample info based on users' needs. > > > > > > But just calling perf_prepare_sample() can be costly when the BPF > > > > So you potentially call it twice now, how's that useful? > > Right. I think we can check data->sample_flags in > perf_prepare_sample() to minimize the duplicate work. > It already does it for some fields, but misses others. we used to have __PERF_SAMPLE_CALLCHAIN_EARLY to avoid extra perf_callchain, could we add some flag like __PERF_SAMPLE_INIT_EARLY to avoid double call to perf_prepare_sample? jirka > > Thanks, > Namhyung
