Cpu perf (#2596)

* break when there's nothing to read

Signed-off-by: Wang, Yi A <yi.a.wang@intel.com>

* Different approach, only listen on stdin when `LOG_LEVEL=debug` (which
is where dropping to a debugger is important).

---------

Signed-off-by: Wang, Yi A <yi.a.wang@intel.com>
Co-authored-by: Wang, Yi A <yi.a.wang@intel.com>

launcher/src/main.rs

@@ -944,6 +944,7 @@ fn shard_manager(
         }
     });
     // We read stdin in another thread as it seems that lines() can block in some cases
+    if LevelFilter::current() >= tracing::Level::DEBUG {
         thread::spawn(move || {
             let mut stdin = io::stdin(); // We get `Stdin` here.
             loop {
@@ -955,6 +956,7 @@ fn shard_manager(
                 }
             }
         });
+    }
 
     let mut ready = false;
     let start_time = Instant::now();