Profiling report broken with foreign exported functions

Save the following Haskell source as wrapper.hs:

import Foreign.Ptr
import Control.Monad

main = do
  fptr <- wrap wrapped
  replicateM 100 $ (return$!) =<< dyn fptr 4 

wrapped :: Double -> IO Double
wrapped x = return $ f 10000 x

f :: Int -> Double -> Double
f 0 u = u
f n u = (u / fromIntegral n) * f (n-1) u

foreign import ccall "wrapper" wrap :: (Double -> IO Double) -> IO (FunPtr (Double -> IO Double))
foreign import ccall "dynamic" dyn :: FunPtr (Double -> IO Double) -> Double -> IO Double

Then compile and run it with:

$ ghc -O2 wrapper.hs -fprof-auto -prof
$ ./wrapper +RTS -p

It generates wrapper.prof (attached). The file contains the following lines:

 CAF        GHC.IO.Encoding.Iconv    58           0    0.0    0.2     0.0    0.2
  wrapped   Main                     80         100    0.0    1.1     0.0    0.0
   f        Main                     81     1000100  100.0    0.0     0.0    0.0

I see two problems here, (1) the inherited column is 0 for 'wrapped' and 'f' but this is incorrect, and (2) 'wrapped' and 'f' belongs to the wrong cost center, 'GHC.IO.Encoding.Iconv'.

Trac metadata

Trac field	Value
Version	7.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Profiling
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import profiling labels

Attached file wrapper.prof (download).

changed milestone to %7.8.1

thanks for the report

I think I know what is happening. The wrapper and f cost centres are actually placed under SYSTEM. I can see it by using +RTS -pa instead of +RTS -p. However SYSTEM is a hidden cost centre, so by default it's not included in the report, resulting in a confusing call tree.

Attached file 0001-rts_apply-uses-CCS_MAIN-rather-than-CCS_SYSTEM-7753.patch (download).

use CCS_MAIN in rts_apply

I'm not sure if this is the best way to fix it, but the attached patch improves the output; now wrapper and f are placed directly under MAIN, and they get inherited costs correctly.

mentioned in commit 773365f9

Merged, thank you Takano!

closed

Merged, thanks!

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added Pnormal label