Re: [RFC perf,bpf 1/5] perf, bpf: Introduce PERF_RECORD_BPF_EVENT

[Peter Zijlstra]

On Wed, Nov 07, 2018 at 06:25:04PM +0000, Song Liu wrote:
> 
> 
> > On Nov 7, 2018, at 12:40 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.