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SubjectRe: [RFC perf,bpf 1/5] perf, bpf: Introduce PERF_RECORD_BPF_EVENT
On Wed, Nov 07, 2018 at 06:25:04PM +0000, Song Liu wrote:
>
>
> > On Nov 7, 2018, at 12:40 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Nov 06, 2018 at 12:52:42PM -0800, Song Liu wrote:
> >> For better performance analysis of BPF programs, this patch introduces
> >> PERF_RECORD_BPF_EVENT, a new perf_event_type that exposes BPF program
> >> load/unload information to user space.
> >>
> >> /*
> >> * Record different types of bpf events:
> >> * enum perf_bpf_event_type {
> >> * PERF_BPF_EVENT_UNKNOWN = 0,
> >> * PERF_BPF_EVENT_PROG_LOAD = 1,
> >> * PERF_BPF_EVENT_PROG_UNLOAD = 2,
> >> * };
> >> *
> >> * struct {
> >> * struct perf_event_header header;
> >> * u16 type;
> >> * u16 flags;
> >> * u32 id; // prog_id or map_id
> >> * };
> >> */
> >> PERF_RECORD_BPF_EVENT = 17,
> >>
> >> PERF_RECORD_BPF_EVENT contains minimal information about the BPF program.
> >> Perf utility (or other user space tools) should listen to this event and
> >> fetch more details about the event via BPF syscalls
> >> (BPF_PROG_GET_FD_BY_ID, BPF_OBJ_GET_INFO_BY_FD, etc.).
> >
> > Why !? You're failing to explain why it cannot provide the full
> > information there.
>
> Aha, I missed this part. I will add the following to next version. Please
> let me know if anything is not clear.

>
> This design decision is picked for the following reasons. First, BPF
> programs could be loaded-and-jited and/or unloaded before/during/after
> perf-record run. Once a BPF programs is unloaded, it is impossible to
> recover details of the program. It is impossible to provide the
> information through a simple key (like the build ID). Second, BPF prog
> annotation is under fast developments. Multiple informations will be
> added to bpf_prog_info in the next few releases. Including all the
> information of a BPF program in the perf ring buffer requires frequent
> changes to the perf ABI, and thus makes it very difficult to manage
> compatibility of perf utility.

So I don't agree with that reasoning. If you want symbol information
you'll just have to commit to some form of ABI. That bpf_prog_info is an
ABI too.

And relying on userspace to synchronously consume perf output to
directly call into the kernel again to get more info (through another
ABI) is a pretty terrible design.

So please try harder. NAK on this.

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