> On Aug 26, 2022, at 2:12 PM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote: > > On Fri, Aug 26, 2022 at 1:59 PM Song Liu <songliubraving@xxxxxx> wrote: >> >> >> >>> On Aug 26, 2022, at 12:30 PM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote: >>> >>> On Fri, Aug 26, 2022 at 11:45 AM Song Liu <songliubraving@xxxxxx> wrote: >>> >>>>> And actually, we can just read ctx->data and get the raw record, >>>>> right..? >>>> >>>> Played with this for a little bit. ctx->data appears to be not >>>> reliable sometimes. I guess (not 100% sure) this is because we >>>> call bpf program before event->orig_overflow_handler. We can >>>> probably add a flag to specify we want to call orig_overflow_handler >>>> first. >>> >>> I'm not sure. The sample_data should be provided by the caller >>> of perf_event_overflow. So I guess the bpf program should see >>> a valid ctx->data. >> >> Let's dig into this. Maybe we need some small changes in >> pe_prog_convert_ctx_access. > > Sure, can you explain the problem in detail and share your program? I push the code to https://git.kernel.org/pub/scm/linux/kernel/git/song/linux.git/log/?h=test-perf-event The code is in tools/bpf/perf-test/. The problem is we cannot get reliable print of data->cpu_entry in /sys/kernel/tracing/trace. > >> >>> Also I want to control calling the orig_overflow_handler based >>> on the return value of the BPF program. So calling the orig >>> handler before BPF won't work for me. :) >> >> Interesting. Could you share more information about the use case? > > Well.. it's nothing new. The bpf_overflow_handler calls the > orig_overflow_handler (which writes the sample to the buffer) > only if the BPF returns non zero. Then I can drop unnecessary > samples based on the sample data by returning 0. > > The possible use cases are > 1. when you want to sample from specific code ranges only > 2. when hardware sets specific bits in raw data I like this idea. We already using BPF in counting perf_event. Now it is time to use it in sampling perf_event. :) Thanks, Song
