Profiling: Allow heap profiling to be controlled dynamically.

This patch exposes three new functions in `GHC.Profiling` which allow
heap profiling to be enabled and disabled dynamically.

1. startHeapProfTimer - Starts heap profiling with the given RTS options
2. stopHeapProfTimer  - Stops heap profiling
3. requestHeapCensus  - Perform a heap census on the next context
                        switch, regardless of whether the timer is enabled or not.

2 files changed, 34 insertions, 1 deletions

diff --git a/libraries/base/GHC/Profiling.hs b/libraries/base/GHC/Profiling.hs
index 917a208b30..b7cef2fecd 100644
--- a/libraries/base/GHC/Profiling.hs
+++ b/libraries/base/GHC/Profiling.hs
@@ -2,7 +2,14 @@
 {-# LANGUAGE NoImplicitPrelude #-}
 
 -- | @since 4.7.0.0
-module GHC.Profiling where
+module GHC.Profiling ( -- * Cost Centre Profiling
+                       startProfTimer
+                     , stopProfTimer
+                       -- * Heap Profiling
+                     , startHeapProfTimer
+                     , stopHeapProfTimer
+                     , requestHeapCensus
+                     )where
 
 import GHC.Base
 
@@ -17,3 +24,26 @@ foreign import ccall stopProfTimer :: IO ()
 --
 -- @since 4.7.0.0
 foreign import ccall startProfTimer :: IO ()
+
+-- | Request a heap census on the next context switch. The census can be
+-- requested whether or not the heap profiling timer is running.
+-- Note: This won't do anything unless you also specify a profiling mode on the
+-- command line using the normal RTS options.
+--
+-- @since 4.16.0.0
+foreign import ccall requestHeapCensus :: IO ()
+
+-- | Start heap profiling. This is called normally by the RTS on start-up,
+-- but can be disabled using the rts flag `--no-automatic-heap-samples`
+-- Note: This won't do anything unless you also specify a profiling mode on the
+-- command line using the normal RTS options.
+--
+-- @since 4.16.0.0
+foreign import ccall startHeapProfTimer :: IO ()
+
+-- | Stop heap profiling.
+-- Note: This won't do anything unless you also specify a profiling mode on the
+-- command line using the normal RTS options.
+--
+-- @since 4.16.0.0
+foreign import ccall stopHeapProfTimer :: IO ()
diff --git a/libraries/base/GHC/RTS/Flags.hsc b/libraries/base/GHC/RTS/Flags.hsc
index 2130ee1aaa..2f161c72c3 100644
--- a/libraries/base/GHC/RTS/Flags.hsc
+++ b/libraries/base/GHC/RTS/Flags.hsc
@@ -288,6 +288,7 @@ data ProfFlags = ProfFlags
     { doHeapProfile            :: DoHeapProfile
     , heapProfileInterval      :: RtsTime -- ^ time between samples
     , heapProfileIntervalTicks :: Word    -- ^ ticks between samples (derived)
+    , startHeapProfileAtStartup :: Bool
     , showCCSOnException       :: Bool
     , maxRetainerSetSize       :: Word
     , ccsLength                :: Word
@@ -584,6 +585,8 @@ getProfFlags = do
             <*> #{peek PROFILING_FLAGS, heapProfileInterval} ptr
             <*> #{peek PROFILING_FLAGS, heapProfileIntervalTicks} ptr
             <*> (toBool <$>
+                  (#{peek PROFILING_FLAGS, startHeapProfileAtStartup} ptr :: IO CBool))
+            <*> (toBool <$>
                   (#{peek PROFILING_FLAGS, showCCSOnException} ptr :: IO CBool))
             <*> #{peek PROFILING_FLAGS, maxRetainerSetSize} ptr
             <*> #{peek PROFILING_FLAGS, ccsLength} ptr