Support ringbuf_query method for Ring buffer

docs/reference_guide.md

@@ -868,6 +868,23 @@ ignores the data associated with the discarded event. Must be preceded by a call
 Examples in situ: <!-- TODO -->
 [search /examples](https://github.com/iovisor/bcc/search?q=ringbuf_submit+path%3Aexamples&type=Code),
 
+### 10. ringbuf_query()
+
+Syntax: ```u64 ringbuf_query(u64 flags)```
+
+Return: Requested value, or 0, if flags are not recognized
+
+Flags:
+ - ```BPF_RB_AVAIL_DATA```: Amount of data not yet consumed
+ - ```BPF_RB_RING_SIZE```: The size of ring buffer
+ - ```BPF_RB_CONS_POS```: Consumer position
+ - ```BPF_RB_PROD_POS```: Producer(s) position
+
+A method of the BPF_RINGBUF_OUTPUT table, for getting various properties of ring buffer. Returned values are momentarily snapshots of ring buffer state and could be off by the time helper returns, so this should be used only for debugging/reporting reasons or for implementing various heuristics, that take into account highly-changeable nature of some of those characteristics.
+
+Examples in situ: <!-- TODO -->
+[search /examples](https://github.com/iovisor/bcc/search?q=ringbuf_query+path%3Aexamples&type=Code),
+
 ## Maps
 
 Maps are BPF data stores, and are the basis for higher level object types including tables, hashes, and histograms.

src/cc/export/helpers.h

@@ -231,6 +231,8 @@ struct _name##_table_t { \
  void (*ringbuf_discard) (void *, u64); \
  /* map.ringbuf_submit(data, flags) */ \
  void (*ringbuf_submit) (void *, u64); \
+ /* map.ringbuf_query(flags) */ \
+ u64 (*ringbuf_query) (u64); \
  u32 max_entries; \
 }; \
 __attribute__((section("maps/ringbuf"))) \

src/cc/frontends/clang/b_frontend_action.cc

@@ -1059,6 +1059,11 @@ bool BTypeVisitor::VisitCallExpr(CallExpr *Call) {
  string args = rewriter_.getRewrittenText(expansionRange(SourceRange(GET_BEGINLOC(Call->getArg(0)),
  GET_ENDLOC(Call->getArg(1)))));
  txt = "bpf_ringbuf_discard(" + args + ")";
+ } else if (memb_name == "ringbuf_query") {
+ string name = string(Ref->getDecl()->getName());
+ string arg0 = rewriter_.getRewrittenText(expansionRange(Call->getArg(0)->getSourceRange()));
+ txt = "bpf_ringbuf_query((void *)bpf_pseudo_fd(1, " + fd + ")";
+ txt += ", " + arg0 + ")";
  } else if (memb_name == "ringbuf_submit") {
  string name = string(Ref->getDecl()->getName());
  string args = rewriter_.getRewrittenText(expansionRange(SourceRange(GET_BEGINLOC(Call->getArg(0)),

tests/python/test_ringbuf.py

@@ -154,5 +154,48 @@ def cb(ctx, data, size):
  self.assertEqual(self.counter, 0)
  b.cleanup()
 
+ @skipUnless(kernel_version_ge(5,8), "requires kernel >= 5.8")
+ def test_ringbuf_query(self):
+ PAGE_SIZE = 8
+ self.counter = 0
+ self.page_counts = 0
+
+ class Data(ct.Structure):
+ _fields_ = [("page_cnt", ct.c_ulonglong)]
+
+ def cb(ctx, data, size):
+ self.assertEqual(size, ct.sizeof(Data))
+ event = ct.cast(data, ct.POINTER(Data)).contents
+ self.page_counts += event.page_cnt
+ self.counter += 1
+
+ text = b"""
+BPF_RINGBUF_OUTPUT(events, %i);
+struct data_t {
+ u64 page_cnt;
+};
+int do_sys_nanosleep(void *ctx) {
+ u64 res = 0;
+ res = events.ringbuf_query(BPF_RB_RING_SIZE);
+ if(res == 0) {
+ return 1;
+ }
+ struct data_t data = {res / PAGE_SIZE};
+ events.ringbuf_output(&data, sizeof(data), 0);
+ return 0;
+}
+"""
+ text = text % PAGE_SIZE
+ b = BPF(text=text)
+ b.attach_kprobe(event=b.get_syscall_fnname(b"nanosleep"),
+ fn_name=b"do_sys_nanosleep")
+ b.attach_kprobe(event=b.get_syscall_fnname(b"clock_nanosleep"),
+ fn_name=b"do_sys_nanosleep")
+ b[b"events"].open_ring_buffer(cb)
+ subprocess.call(['sleep', '0.1'])
+ b.ring_buffer_poll()
+ self.assertEqual(self.page_counts / self.counter, PAGE_SIZE)
+ b.cleanup()
+
 if __name__ == "__main__":
  main()