[tor-bugs] #24668 [Core Tor/Tor]: sched: scheduler_compare_channels() function will never pick a channel with no active circuis.

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#24668: sched: scheduler_compare_channels() function will never pick a channel with
no active circuis.
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     Reporter:  dgoulet     |      Owner:  (none)
         Type:  defect      |     Status:  new
     Priority:  Medium      |  Milestone:  Tor: 0.3.3.x-final
    Component:  Core        |    Version:
  Tor/Tor                   |
     Severity:  Normal      |   Keywords:  tor-sched, tor-channel, tor-cell
Actual Points:              |  Parent ID:
       Points:              |   Reviewer:
      Sponsor:              |
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 In the schedulers, scheduler_compare_channels() is used to decide which
 channel is 'best' to write data from.  It delegates to
 circuitmux_compare_muxes(), which delegates to ewma_cmp_cmux().

 But ewma_cmp_cmux() will never prefer a cmux with no active circuits on
 it!  So a channel without active circuits will never be picked by the
 scheduler to flush from a circuit, which is what triggers flushing from
 its destroy queue.  So the channel will stay around forever, never
 flushing.

 To fix this one, we probably have to fix ewma_cmp_cmux() to look at
 destroy cells too (somehow).  And we still need to make sure that the
 scheduler's position in the heap changes when the data considered by
 scheduler_compare_channels() changes [*].

 [*] I'm not convinced that we're even doing this right with the current
 scheduler_compare_channels() code. :(

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24668>
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