diff --git a/xprof/xprof.rb.in b/xprof/xprof.rb.in
index 9b4804dd..8c2be45c 100755
--- a/xprof/xprof.rb.in
+++ b/xprof/xprof.rb.in
@@ -917,9 +917,14 @@ def all_env_tracers(usr_binary)
     h["LTTNG_UST_#{name.upcase}_PROFILE"] = 1 if OPTIONS[:profile]
     h["LTTNG_UST_#{name.upcase}_VERBOSE"] = 1 if LOGGER.level <= Logger::DEBUG
   end
-  # Only support blocking.
+  # Only support blocking, so lttng doesn't drop messages
   h['LTTNG_UST_ALLOW_BLOCKING'] = 1
 
+  # Each tracee rank tries to register simultaneously.
+  # With the default (3s) timeout, ranks that lose the registration race give up
+  # and run untraced. Block until registration completes instead.
+  h['LTTNG_UST_REGISTER_TIMEOUT'] = -1
+
   # Customization
   if OPTIONS[:'backend-names'].include?('ze')
     h['LTTNG_UST_ZE_PARANOID_DRIFT'] = 1 if OPTIONS[:profile]