On Thu, Aug 25, 2022 at 7:35 PM Song Liu <songliubraving@xxxxxx> wrote: > > > > > On Aug 25, 2022, at 4:03 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > > > On Thu, Aug 25, 2022 at 3:08 PM Song Liu <songliubraving@xxxxxx> wrote: > >> > >> > >> > >>> On Aug 25, 2022, at 2:33 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > >>> > >>> On Tue, Aug 23, 2022 at 2:04 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote: > >>>> + * long bpf_read_raw_record(struct bpf_perf_event_data *ctx, void *buf, u32 size, u64 flags) > >>>> + * Description > >>>> + * For an eBPF program attached to a perf event, retrieve the > >>>> + * raw record associated to *ctx* and store it in the buffer > >>>> + * pointed by *buf* up to size *size* bytes. > >>>> + * Return > >>>> + * On success, number of bytes written to *buf*. On error, a > >>>> + * negative value. > >>>> + * > >>>> + * The *flags* can be set to **BPF_F_GET_RAW_RECORD_SIZE** to > >>>> + * instead return the number of bytes required to store the raw > >>>> + * record. If this flag is set, *buf* may be NULL. > >>> > >>> It looks pretty ugly from a usability standpoint to have one helper > >>> doing completely different things and returning two different values > >>> based on BPF_F_GET_RAW_RECORD_SIZE. > >> > >> Yeah, I had the same thought when I first looked at it. But that's the > >> exact syntax with bpf_read_branch_records(). Well, we still have time > >> to fix the new helper.. > >> > >>> > >>> I'm not sure what's best, but I have two alternative proposals: > >>> > >>> 1. Add two helpers: one to get perf record information (and size will > >>> be one of them). Something like bpf_perf_record_query(ctx, flags) > >>> where you pass perf ctx and what kind of information you want to read > >>> (through flags), and u64 return result returns that (see > >>> bpf_ringbuf_query() for such approach). And then have separate helper > >>> to read data. > >>> > >>> 2. Keep one helper, but specify that it always returns record size, > >>> even if user specified smaller size to read. And then allow passing > >>> buf==NULL && size==0. So passing NULL, 0 -- you get record size. > >>> Passing non-NULL buf -- you read data. > >> > >> AFAICT, this is also confusing. > >> > > > > this is analogous to snprintf() behavior, so not that new and > > surprising when you think about it. But if query + read makes more > > sense, then it's fine by me > > Given the name discussion (the other email), I now like one API better. > > Actually, since we are on this, can we make it more generic, and handle > all possible PERF_SAMPLE_* (in enum perf_event_sample_format)? Something > like: > > long bpf_perf_event_read_sample(void *ctx, void *buf, u64 size, u64 flags); > > WDYT Namhyung? Do you mean reading the whole sample data at once? Then it needs to parse the sample data format properly which is non trivial due to a number of variable length fields like callchains and branch stack, etc. Also I'm afraid I might need event configuration info other than sample data like attr.type, attr.config, attr.sample_type and so on. Hmm.. maybe we can add it to the ctx directly like ctx.attr_type? > > Another idea is to add another parameter, so that we can pick which > PERF_SAMPLE_* to output via bpf_perf_event_read_sample(). > > I think this will cover all cases with sample perf_event. Thoughts? Yeah, I like this more and it looks easier to use. Thanks, Namhyung
