Re: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read_sample() helper
From: Song Liu
Date: Tue Nov 01 2022 - 16:04:28 EST
On Tue, Nov 1, 2022 at 11:53 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Tue, Nov 1, 2022 at 11:47 AM Song Liu <song@xxxxxxxxxx> wrote:
> >
> > On Tue, Nov 1, 2022 at 11:26 AM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > On Tue, Nov 1, 2022 at 3:03 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> > > >
> > > > On Mon, Oct 31, 2022 at 10:23:39PM -0700, Namhyung Kim wrote:
> > > > > The bpf_perf_event_read_sample() helper is to get the specified sample
> > > > > data (by using PERF_SAMPLE_* flag in the argument) from BPF to make a
> > > > > decision for filtering on samples. Currently PERF_SAMPLE_IP and
> > > > > PERF_SAMPLE_DATA flags are supported only.
> > > > >
> > > > > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > > > > ---
> > > > > include/uapi/linux/bpf.h | 23 ++++++++++++++++
> > > > > kernel/trace/bpf_trace.c | 49 ++++++++++++++++++++++++++++++++++
> > > > > tools/include/uapi/linux/bpf.h | 23 ++++++++++++++++
> > > > > 3 files changed, 95 insertions(+)
> > > > >
> > > > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > > > index 94659f6b3395..cba501de9373 100644
> > > > > --- a/include/uapi/linux/bpf.h
> > > > > +++ b/include/uapi/linux/bpf.h
> > > > > @@ -5481,6 +5481,28 @@ union bpf_attr {
> > > > > * 0 on success.
> > > > > *
> > > > > * **-ENOENT** if the bpf_local_storage cannot be found.
> > > > > + *
> > > > > + * long bpf_perf_event_read_sample(struct bpf_perf_event_data *ctx, void *buf, u32 size, u64 sample_flags)
> > > > > + * Description
> > > > > + * For an eBPF program attached to a perf event, retrieve the
> > > > > + * sample data associated to *ctx* and store it in the buffer
> > > > > + * pointed by *buf* up to size *size* bytes.
> > > > > + *
> > > > > + * The *sample_flags* should contain a single value in the
> > > > > + * **enum perf_event_sample_format**.
> > > > > + * Return
> > > > > + * On success, number of bytes written to *buf*. On error, a
> > > > > + * negative value.
> > > > > + *
> > > > > + * The *buf* can be set to **NULL** to return the number of bytes
> > > > > + * required to store the requested sample data.
> > > > > + *
> > > > > + * **-EINVAL** if *sample_flags* is not a PERF_SAMPLE_* flag.
> > > > > + *
> > > > > + * **-ENOENT** if the associated perf event doesn't have the data.
> > > > > + *
> > > > > + * **-ENOSYS** if system doesn't support the sample data to be
> > > > > + * retrieved.
> > > > > */
> > > > > #define ___BPF_FUNC_MAPPER(FN, ctx...) \
> > > > > FN(unspec, 0, ##ctx) \
> > > > > @@ -5695,6 +5717,7 @@ union bpf_attr {
> > > > > FN(user_ringbuf_drain, 209, ##ctx) \
> > > > > FN(cgrp_storage_get, 210, ##ctx) \
> > > > > FN(cgrp_storage_delete, 211, ##ctx) \
> > > > > + FN(perf_event_read_sample, 212, ##ctx) \
> > > > > /* */
> > > > >
> > > > > /* backwards-compatibility macros for users of __BPF_FUNC_MAPPER that don't
> > > > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > > > index ce0228c72a93..befd937afa3c 100644
> > > > > --- a/kernel/trace/bpf_trace.c
> > > > > +++ b/kernel/trace/bpf_trace.c
> > > > > @@ -28,6 +28,7 @@
> > > > >
> > > > > #include <uapi/linux/bpf.h>
> > > > > #include <uapi/linux/btf.h>
> > > > > +#include <uapi/linux/perf_event.h>
> > > > >
> > > > > #include <asm/tlb.h>
> > > > >
> > > > > @@ -1743,6 +1744,52 @@ static const struct bpf_func_proto bpf_read_branch_records_proto = {
> > > > > .arg4_type = ARG_ANYTHING,
> > > > > };
> > > > >
> > > > > +BPF_CALL_4(bpf_perf_event_read_sample, struct bpf_perf_event_data_kern *, ctx,
> > > > > + void *, buf, u32, size, u64, flags)
> > > > > +{
> > > >
> > > > I wonder we could add perf_btf (like we have tp_btf) program type that
> > > > could access ctx->data directly without helpers
> > > >
> > > > > + struct perf_sample_data *sd = ctx->data;
> > > > > + void *data;
> > > > > + u32 to_copy = sizeof(u64);
> > > > > +
> > > > > + /* only allow a single sample flag */
> > > > > + if (!is_power_of_2(flags))
> > > > > + return -EINVAL;
> > > > > +
> > > > > + /* support reading only already populated info */
> > > > > + if (flags & ~sd->sample_flags)
> > > > > + return -ENOENT;
> > > > > +
> > > > > + switch (flags) {
> > > > > + case PERF_SAMPLE_IP:
> > > > > + data = &sd->ip;
> > > > > + break;
> > > > > + case PERF_SAMPLE_ADDR:
> > > > > + data = &sd->addr;
> > > > > + break;
> > > >
> > > > AFAICS from pe_prog_convert_ctx_access you should be able to read addr
> > > > directly from context right? same as sample_period.. so I think if this
> > > > will be generic way to read sample data, should we add sample_period
> > > > as well?
> > >
> > > +1
> > > Let's avoid new stable helpers for this.
> > > Pls use CORE and read perf_sample_data directly.
> >
> > We have legacy ways to access sample_period and addr with
> > struct bpf_perf_event_data and struct bpf_perf_event_data_kern. I
> > think mixing that
> > with CORE makes it confusing for the user. And a helper or a kfunc would make it
> > easier to follow. perf_btf might also be a good approach for this.
>
> imo that's a counter argument to non-CORE style.
> struct bpf_perf_event_data has sample_period and addr,
> and as soon as we pushed the boundaries it turned out it's not enough.
> Now we're proposing to extend uapi a bit with sample_ip.
> That will repeat the same mistake.
> Just use CORE and read everything that is there today
> and will be there in the future.
Another work of this effort is that we need the perf_event to prepare
required fields before calling the BPF program. I think we will need
some logic in addition to CORE to get that right. How about we add
perf_btf where the perf_event prepare all fields before calling the
BPF program? perf_btf + CORE will be able to read all fields in the
sample.
Thanks,
Song