bpf: improve verification speed by not remarking live_read

With large verifier speed improvement brought by the previous patch
mark_reg_read() becomes the hottest function during verification.
On a typical program it consumes 40% of cpu.
mark_reg_read() walks parentage chain of registers to mark parents as LIVE_READ.
Once the register is marked there is no need to remark it again in the future.
Hence stop walking the chain once first LIVE_READ is seen.
This optimization drops mark_reg_read() time from 40% of cpu to <1%
and overall 2x improvement of verification speed.
For some programs the longest_mark_read_walk counter improves from ~500 to ~5

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/verifier.c

@@ -1151,6 +1151,15 @@ static int mark_reg_read(struct bpf_verifier_env *env,
 				parent->var_off.value, parent->off);
 			return -EFAULT;
 		}
+		if (parent->live & REG_LIVE_READ)
+			/* The parentage chain never changes and
+			 * this parent was already marked as LIVE_READ.
+			 * There is no need to keep walking the chain again and
+			 * keep re-marking all parents as LIVE_READ.
+			 * This case happens when the same register is read
+			 * multiple times without writes into it in-between.
+			 */
+			break;
 		/* ... then we depend on parent's value */
 		parent->live |= REG_LIVE_READ;
 		state = parent;