Re: [PATCH v3 perf, bpf-next 1/4] perf, bpf: Introduce PERF_RECORD_BPF_EVENT

On Dec 12, 2018, at 5:15 AM, Peter Zijlstra [off-list ref] wrote:

On Tue, Dec 11, 2018 at 03:33:47PM -0800, Song Liu wrote:

quoted

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.

Each BPF program may contain up to BPF_MAX_SUBPROGS (256) sub programs.
The following example shows kernel symbols for a BPF program with 7
sub programs:

   ffffffffa0257cf9 t bpf_prog_b07ccb89267cf242_F
   ffffffffa02592e1 t bpf_prog_2dcecc18072623fc_F
   ffffffffa025b0e9 t bpf_prog_bb7a405ebaec5d5c_F
   ffffffffa025dd2c t bpf_prog_a7540d4a39ec1fc7_F
   ffffffffa025fcca t bpf_prog_05762d4ade0e3737_F
   ffffffffa026108f t bpf_prog_db4bd11e35df90d4_F
   ffffffffa0263f00 t bpf_prog_89d64e4abf0f0126_F
   ffffffffa0257cf9 t bpf_prog_ae31629322c4b018__dummy_tracepoi

Doesn't BPF have enough information to generate 'saner' names? Going by
the thing below, these sub-progs are actually functions, right?

These sub programs/functions will have their descriptive names from BTF
function types (coming in 4.21). However, BTF is optional in normal cases, 
when BTF is missing, they will be named as bpf_prog_<tag>_F. The main BPF
program has a name up to 16 byte long. In the example above, the last 
program has name _dummy_tracepoint. 

I think these sub programs are more like "programs" than "functions", 
because each sub program occupies its own page(s). 

quoted

       /*
        * 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;
        *      u32                             type;
        *      u32                             flags;
        *      u32                             id; // prog_id or other id
        *      u32                             sub_id; // subprog id
        *
        *      // for bpf_prog types, bpf prog or subprog
        *      u8                              tag[BPF_TAG_SIZE];
        *      u64                             addr;
        *      u64                             len;
        *      char                            name[];
        *      struct sample_id                sample_id;
        * };
        */

Isn't this mixing two different things (poorly)? The kallsym update and
the BPF load/unload ?

I would say these two things are actually two parts of the same event. 
Fields id, sub_id, and tag provide information about which program is
mapped to this ksym. They are equivalent to "pgoff + filename" in 
PERF_RECORD_MMAP, or "maj, min, ino, and ino_generation" in 
PERF_RECORD_MMAP2. 

And while this tracks the bpf kallsyms, it does not do all kallsyms.

.... Oooh, I see the problem, everybody is doing their own custom
kallsym_{add,del}() thing, instead of having that in generic code :-(

This, for example, doesn't track module load/unload nor ftrace
trampolines, even though both affect kallsyms.

I think we can use PERF_RECORD_MMAP(or MMAP2) for module load/unload. 
That could be separate sets of patches. 

Thanks,
Song

quoted

+void perf_event_bpf_event_prog(enum perf_bpf_event_type type,
+			       struct bpf_prog *prog)
+{
+	if (!atomic_read(&nr_bpf_events))
+		return;
+
+	if (type != PERF_BPF_EVENT_PROG_LOAD &&
+	    type != PERF_BPF_EVENT_PROG_UNLOAD)
+		return;
+
+	if (prog->aux->func_cnt == 0) {
+		perf_event_bpf_event_subprog(type, prog,
+					     prog->aux->id, 0);
+	} else {
+		int i;
+
+		for (i = 0; i < prog->aux->func_cnt; i++)
+			perf_event_bpf_event_subprog(type, prog->aux->func[i],
+						     prog->aux->id, i);
+	}
+}