[GHC] #15880: GHC.Stats: Add info on amount of pinned memory

[GHC]

#15880: GHC.Stats: Add info on amount of pinned memory
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           Reporter:  nh2            |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Runtime        |           Version:  8.6.2
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 https://hackage.haskell.org/package/base-4.12.0.0/docs/GHC-Stats.html

 `GHC.Stats`'s `GCDetails` tells us, separately, the amount of

 * total heap data (including large objects and compact data)
 * large ojects
 * compact data
 * "Total amount of memory in use by the RTS"
 (`gcdetails_mem_in_use_bytes`)

 There currently doesn't seem to be a way to get the amount of pinned
 memory in use.

 So some questions:

 * Is pinned memory accounted for in `gcdetails_mem_in_use_bytes`?
 * If yes, is it pretty much that value minus `gcdetails_live_bytes`, or
 are there other memory uses that count to the latter?
 * If I wanted to add the amount of pinned memory directly to `GHC.Stats`,
 where would I take that value from?

 In general, my goal is to collect _all_ easily (cheaply) collectable
 memory info so that I can add it to `ekg`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15880>
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