[GHC] #12419: Scheduling bug with forkOS + MVar

[GHC]

#12419: Scheduling bug with forkOS + MVar
-------------------------------------+-------------------------------------
        Reporter:  luisgabriel       |                Owner:
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Runtime System    |              Version:  8.0.1
      Resolution:                    |             Keywords:  forkOS;
                                     |  scheduler
Operating System:  Linux             |         Architecture:  x86_64
                                     |  (amd64)
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2430,
       Wiki Page:                    |  Phab:D2441
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"9f1b6de8b3f788730fa5e2206fb709400299be7c/ghc"
 9f1b6de8/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="9f1b6de8b3f788730fa5e2206fb709400299be7c"
 Fix to thread migration

 Summary:
 If we had 2 threads on the run queue, say [A,B], and B is bound to the
 current Task, then we would fail to migrate any threads.  This fixes it
 so that we would migrate A in that case.

 This will help parallelism a bit in programs that have lots of bound
 threads.

 Test Plan:
 Test program in #12419, which is actually not a great program but it
 does behave a bit better after this change.

 Reviewers: ezyang, niteria, bgamari, austin, erikd

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2430

 GHC Trac Issues: #12419

 (cherry picked from commit 988ad8ba8e709eff3cea59728e481bb269fa6185)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12419#comment:13>
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