Re: [PATCH v4 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot

[Andrii Nakryiko]

On Wed, Sep 1, 2021 at 8:41 AM Song Liu <songliubraving@xxxxxx> wrote:
>
>
>
> > On Aug 31, 2021, at 9:02 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Tue, Aug 31, 2021 at 7:01 PM Song Liu <songliubraving@xxxxxx> wrote:
> >>
> >> Introduce bpf_get_branch_snapshot(), which allows tracing pogram to get
> >> branch trace from hardware (e.g. Intel LBR). To use the feature, the
> >> user need to create perf_event with proper branch_record filtering
> >> on each cpu, and then calls bpf_get_branch_snapshot in the bpf function.
> >> On Intel CPUs, VLBR event (raw event 0x1b00) can be use for this.
> >>
> >> Signed-off-by: Song Liu <songliubraving@xxxxxx>
> >> ---
> >> include/uapi/linux/bpf.h       | 22 +++++++++++++++++++
> >> kernel/bpf/trampoline.c        |  3 ++-
> >> kernel/trace/bpf_trace.c       | 40 ++++++++++++++++++++++++++++++++++
> >> tools/include/uapi/linux/bpf.h | 22 +++++++++++++++++++
> >> 4 files changed, 86 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> >> index 791f31dd0abee..c986e6fad5bc0 100644
> >> --- a/include/uapi/linux/bpf.h
> >> +++ b/include/uapi/linux/bpf.h
> >> @@ -4877,6 +4877,27 @@ union bpf_attr {
> >>  *             Get the struct pt_regs associated with **task**.
> >>  *     Return
> >>  *             A pointer to struct pt_regs.
> >> + *
> >> + * long bpf_get_branch_snapshot(void *entries, u32 size, u64 flags)
> >> + *     Description
> >> + *             Get branch trace from hardware engines like Intel LBR. The
> >> + *             branch trace is taken soon after the trigger point of the
> >> + *             BPF program, so it may contain some entries after the
> >> + *             trigger point. The user need to filter these entries
> >> + *             accordingly.
> >> + *
> >> + *             The data is stored as struct perf_branch_entry into output
> >> + *             buffer *entries*. *size* is the size of *entries* in bytes.
> >> + *             *flags* is reserved for now and must be zero.
> >> + *
> >> + *     Return
> >> + *             On success, number of bytes written to *buf*. On error, a
> >> + *             negative value.
> >> + *
> >> + *             **-EINVAL** if arguments invalid or **size** not a multiple
> >> + *             of **sizeof**\ (**struct perf_branch_entry**\ ).
> >> + *
> >> + *             **-ENOENT** if architecture does not support branch records.
> >>  */
> >> #define __BPF_FUNC_MAPPER(FN)          \
> >>        FN(unspec),                     \
> >> @@ -5055,6 +5076,7 @@ union bpf_attr {
> >>        FN(get_func_ip),                \
> >>        FN(get_attach_cookie),          \
> >>        FN(task_pt_regs),               \
> >> +       FN(get_branch_snapshot),        \
> >>        /* */
> >>
> >> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> >> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> >> index fe1e857324e66..39eaaff81953d 100644
> >> --- a/kernel/bpf/trampoline.c
> >> +++ b/kernel/bpf/trampoline.c
> >> @@ -10,6 +10,7 @@
> >> #include <linux/rcupdate_trace.h>
> >> #include <linux/rcupdate_wait.h>
> >> #include <linux/module.h>
> >> +#include <linux/static_call.h>
> >>
> >> /* dummy _ops. The verifier will operate on target program's ops. */
> >> const struct bpf_verifier_ops bpf_extension_verifier_ops = {
> >> @@ -526,7 +527,7 @@ void bpf_trampoline_put(struct bpf_trampoline *tr)
> >> }
> >>
> >> #define NO_START_TIME 1
> >> -static u64 notrace bpf_prog_start_time(void)
> >> +static __always_inline u64 notrace bpf_prog_start_time(void)
> >> {
> >>        u64 start = NO_START_TIME;
> >>
> >> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> >> index 8e2eb950aa829..a8ec3634a3329 100644
> >> --- a/kernel/trace/bpf_trace.c
> >> +++ b/kernel/trace/bpf_trace.c
> >> @@ -1017,6 +1017,44 @@ static const struct bpf_func_proto bpf_get_attach_cookie_proto_pe = {
> >>        .arg1_type      = ARG_PTR_TO_CTX,
> >> };
> >>
> >> +static DEFINE_PER_CPU(struct perf_branch_snapshot, bpf_perf_branch_snapshot);
> >> +
> >> +BPF_CALL_3(bpf_get_branch_snapshot, void *, buf, u32, size, u64, flags)
> >> +{
> >> +#ifndef CONFIG_X86
> >> +       return -ENOENT;
> >
> > nit: -EOPNOTSUPP probably makes more sense for this?
>
> I had -EOPNOTSUPP in earlier version. But bpf_read_branch_records uses
> -ENOENT, so I updated here in v4. I guess -ENOENT also makes sense? I
> won't insist if you think -EOPNOTSUPP is better.

Hm... ok, I guess consistency takes priority, let's keep -ENOENT then.

>
> >
> >> +#else
> >> +       static const u32 br_entry_size = sizeof(struct perf_branch_entry);
> >> +       u32 to_copy;
> >> +
> >> +       if (unlikely(flags))
> >> +               return -EINVAL;
> >> +
> >> +       if (!buf || (size % br_entry_size != 0))
> >> +               return -EINVAL;
> >> +
> >> +       static_call(perf_snapshot_branch_stack)(this_cpu_ptr(&bpf_perf_branch_snapshot));
> >
> > First, you have four this_cpu_ptr(&bpf_perf_branch_snapshot)
> > invocations in this function, probably cleaner to store the pointer in
> > local variable?
> >
> > But second, this still has the reentrancy problem, right? And further,
> > we copy the same LBR data twice (to per-cpu buffer and into
> > user-provided destination).
> >
> > What if we change perf_snapshot_branch_stack signature to this:
> >
> > int perf_snapshot_branch_stack(struct perf_branch_entry *entries, int
> > max_nr_entries);
> >
> > with the semantics that it will copy only min(max_nr_entreis,
> > PERF_MAX_BRANCH_RECORDS) * sizeof(struct perf_branch_entry) bytes.
> > That way we can copy directly into a user-provided buffer with no
> > per-cpu storage. Of course, perf_snapshot_branch_stack will return
> > number of entries copied, either as return result, or if static calls
> > don't support that, as another int *nr_entries output argument.
>
> I like this idea. Once we get feedback from Peter, I will change this
> in v5.

Sounds good, thanks!

>
> Thanks,
> Song
>