Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper

[Song Liu]

On Thu, Aug 25, 2022 at 10:22 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> 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?

The user should have access to the perf_event_attr used to
create the event. This is also available in ctx->event->attr.

Would this work?

Thanks,
Song

>
> >
> > 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.