Various　datacenter　network　(DCN)　load　balancing　schemes　have　been　proposed　in　the　past　decade.　Unfortunately,　most　of　these　solutions　designed　for　lossy　DCNs　do　not　work　well　for　Priority　Flow　Control　(PFC)　enabled　lossless　DCNs,　primarily　due　to　the　reason　that　the　individual　congestion　signals　used　in　these　solutions,　e.g.,　link　load,　queue　length,　Round　Trip　Time　(RTT)　and　Explicit　Congestion　Notification　(ECN),　may　not　be　able　to　correctly　or　timely　reflect　the　hop-by-hop　PFC　pausing.　This　paper　first　reveals　the　above　problems　via　extensive　experiments,　and　then　based　on　the　insights　learned,　we　present　Proteus,　a　PFC-aware　load　balancing　scheme　that　is　resilient　to　PFC　pausing　by　exploring　a　combination　of　multi-level　congestion　signals.　At　its　heart,　Proteus　leverages　RTT-level　signals　(i.e.,　RTT　and　link　utilization)　to　detect　path　status　for　initial　routing　decision,　and　exploits　sub-RTT　level　signal　(i.e.,　cumulative　sojourn　time)　to　reflect　instantaneous　PFC　pausing　and　make　timely　rerouting　choices　based　on　the　idea　of　better-late-than-never.　We　have　implemented　Proteus　in　the　hardware　programmable　switch.　Our　testbed　experiments　as　well　as　large-scale　simulations　show　that　Proteus　can　effectively　handle　PFC　pausing　under　realistic　workloads　and　achieve　up　to　35%,　31%,　28%,　22%　and　46%,　42%,　34%,　29%　better　average　FCT　and　　$99^{th}$　　percentile　FCT　than　CONGA,　DRILL,　Hermes　and　MP-RDMA,　respectively.　IEEE