main event-loop spins consuming 100% of a CPU core running scheduler_set_channel_state

Lately have observed my exit hitting 100% cpu on the main even-loop thread, sometimes continuously, sometimes cyclically. Captured full-debug of recent cyclical event where CPU started at 30% and rose to 100%, for about one cycle. Chopped the 1G log into eight slices and took a simple function call-count histogram. What's notable is not an increase of calls during saturation, but a reduction of several that seem to relate to connection close events (conn_close_if_marked, flush_chunk). Left column is for the first slice where CPU was 30%, right column is for fourth slice where cpu was 100%. Functions with less than 1000 calls not included below, but complete histograms attached. Wrote about this on tor-relays:

https://lists.torproject.org/pipermail/tor-relays/2018-March/014730.html

This might be an attack of some kind, or perhaps a misbehavior related to the KIST scheduler.

append_cell_to_circuit_queue 6787                 append_cell_to_circuit_queue 7280
channel_flush_from_first_active_circuit 6781      channel_flush_from_first_active_circuit 7190
channel_process_cell 11904                        channel_process_cell 11813
channel_write_packed_cell 120301                  channel_write_packed_cell 126330
channel_write_to_kernel 8588                      channel_write_to_kernel 10048
circuit_consider_stop_edge_reading 146965         circuit_consider_stop_edge_reading 152665
circuit_get_by_circid_channel_impl 14128          circuit_get_by_circid_channel_impl 13468
circuit_receive_relay_cell 11483                  circuit_receive_relay_cell 11341
circuit_resume_edge_reading 1203                  circuit_resume_edge_reading 1231
conn_close_if_marked 39033                        conn_close_if_marked 779
conn_read_callback 14743                          conn_read_callback 15645
conn_write_callback 4531                          conn_write_callback 4447
connection_add_impl 1023                          connection_add_impl 739
connection_bucket_refill_helper 14787             connection_bucket_refill_helper 15842
connection_buf_read_from_socket 16196             connection_buf_read_from_socket 17152
connection_connect 1016                           connection_connect 732
connection_connect_sockaddr 1016                  connection_connect_sockaddr 732
connection_edge_package_raw_inbuf 237303          connection_edge_package_raw_inbuf 255347
connection_edge_process_relay_cell 22219          connection_edge_process_relay_cell 22332
connection_exit_begin_conn 3165                   connection_exit_begin_conn 2315
connection_exit_connect 1050                      connection_exit_connect 772
connection_handle_write_impl 9240                 connection_handle_write_impl 10539
connection_or_process_cells_from_inbuf 20042      connection_or_process_cells_from_inbuf 20448
flush_chunk 38192                                 flush_chunk 12
flush_chunk_tls 22283                             flush_chunk_tls 24061
free_outbuf_info_by_ent 8588                      free_outbuf_info_by_ent 10047
outbuf_table_add 8588                             outbuf_table_add 10014
read_to_chunk 6856                                read_to_chunk 7254
relay_lookup_conn 8459                            relay_lookup_conn 8525
relay_send_command_from_edge_ 119963              relay_send_command_from_edge_ 128738
rep_hist_note_exit_bytes 13913                    rep_hist_note_exit_bytes 14534
scheduler_set_channel_state 126896                scheduler_set_channel_state 133353
update_socket_info 6719                           update_socket_info 7160
update_socket_written 120297                      update_socket_written 126327