[tor-bugs] #25461 [Core Tor/Tor]: main event-loop spins consuming 100% of a CPU core running scheduler_set_channel_state

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#25461: main event-loop spins consuming 100% of a CPU core running
scheduler_set_channel_state
-------------------------------------------------+-------------------------
 Reporter:  Dhalgren                             |          Owner:  (none)
     Type:  defect                               |         Status:
                                                 |  assigned
 Priority:  High                                 |      Milestone:  Tor:
                                                 |  unspecified
Component:  Core Tor/Tor                         |        Version:  Tor:
                                                 |  0.2.9.14
 Severity:  Normal                               |     Resolution:
 Keywords:  performance, 034-roadmap-subtask,    |  Actual Points:
  034-triage-20180328, 034-included-20180328     |
  035-removed                                    |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
-------------------------------------------------+-------------------------

Comment (by arma):

 I just hit this bug on my tor client:
 {{{
 Tor 0.4.1.0-alpha-dev (git-aee65084b421d0b5) running on Linux with
 Libevent 2.1.8-stable, OpenSSL 1.1.1b, Zlib 1.2.11, Liblzma N/A, and
 Libzstd N/A.
 }}}

 I have this clause over and over and over in my debug-level logs:
 {{{
 May 12 23:55:38.642 [debug] scheduler_evt_callback(): Scheduler event
 callback called
 May 12 23:55:38.642 [debug] update_socket_info(): chan=1 updated socket
 info, limit: 0, cwnd: 1, unacked: 3, notsent: 29865, mss: 1448
 May 12 23:55:38.642 [debug] kist_scheduler_run(): Running the scheduler. 1
 channels pending
 May 12 23:55:38.642 [debug] outbuf_table_add(): scheduler init outbuf info
 for chan=1
 May 12 23:55:38.642 [debug] channel_write_to_kernel(): Writing 0 bytes to
 kernel for chan 1
 May 12 23:55:38.642 [debug] flush_chunk_tls(): flushed 0 bytes, 0 ready to
 flush, 0 remain.
 May 12 23:55:38.642 [debug] connection_handle_write_impl(): After TLS
 write of 0: 0 read, 0 written
 May 12 23:55:38.642 [debug] free_outbuf_info_by_ent(): Freeing outbuf
 table entry from chan=1
 May 12 23:55:38.642 [debug] kist_scheduler_run(): len pending=0, len
 to_readd=1
 May 12 23:55:38.642 [debug] scheduler_set_channel_state(): chan 1 changed
 from scheduler state PENDING to PENDING
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25461#comment:24>
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