[GHC] #15897: Negative MUT time in +RTS -s -RTS when heap profiling is enabled

[GHC]

#15897: Negative MUT time in +RTS -s -RTS when heap profiling is enabled
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        Reporter:  maoe              |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Profiling         |              Version:  8.6.2
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Debugging         |  (amd64)
  information is incorrect           |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Ömer Sinan Ağacan <omeragacan@…>):

 In [changeset:"19670bc397d858b04eb9b4eb01480f7f8c59e2f5/ghc"
 19670bc3/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="19670bc397d858b04eb9b4eb01480f7f8c59e2f5"
 Fix negative mutator time in GC stats in prof builds

 Because garbage collector calls `retainerProfile()` and `heapCensus()`,
 GC times normally include some of PROF times too. To fix this we have
 these lines:

     // heapCensus() is called by the GC, so RP and HC time are
     // included in the GC stats.  We therefore subtract them to
     // obtain the actual GC cpu time.
     stats.gc_cpu_ns      -=  prof_cpu;
     stats.gc_elapsed_ns  -=  prof_elapsed;

 These variables are later used for calculating GC time excluding the
 final GC (which should be attributed to EXIT).

     exit_gc_elapsed      = stats.gc_elapsed_ns - start_exit_gc_elapsed;

 The problem is if we subtract PROF times from `gc_elapsed_ns` and then
 subtract `start_exit_gc_elapsed` from the result, we end up subtracting
 PROF times twice, because `start_exit_gc_elapsed` also includes PROF
 times.

 We now subtract PROF times from GC after the calculations for EXIT and
 MUT times. The existing assertion that checks

     INIT + MUT + GC + EXIT = TOTAL

 now holds. When we subtract PROF numbers from GC, and a new assertion

     INIT + MUT + GC + PROF + EXIT = TOTAL

 also holds.

 Fixes #15897. New assertions added in this commit also revealed #16102,
 which is also fixed by this commit.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15897#comment:10>
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