[GHC] #16143: Some cost centre stacks are not registered via registerCcsList()

7 Jan 2019

      #16143: Some cost centre stacks are not registered via registerCcsList()
-------------------------------------+-------------------------------------
           Reporter:  osa1           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Profiling      |           Version:  8.6.3
           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:
-------------------------------------+-------------------------------------
 In #15508 I observe that some closures have incorrect CCS field. To be
 able to
 catch this problem as early as possible I implemented a simple check:

 {{{
 static void
 checkClosureProfSanity(const StgClosure *p)
 {
     StgProfHeader prof_hdr = p->header.prof;
     CostCentreStack *ccs = prof_hdr.ccs;

     // For now just make sure the CCS is a registered CCS and not a
 pointer to
     // some random heap closure (happened before, see #15508).
     for (CostCentreStack *ccs_ = CCS_LIST; ccs_; ccs_ = ccs_->prevStack) {
         if (ccs == ccs_) {
             return;
         }
     }

     barf("CCS for closure %p is not registered: %p", (void*)p,
 (void*)ccs);
 }
 }}}

 I call this on all closures before and after a GC. This assumes that all
 cost
 centre stacks are registered via `registerCcsList()`, however this
 currently
 does not hold. I was able to find two cases:

 - `base_GHCziIOziHandleziFD_CAFs_cc_ccs` is never registered (there are
 probably
   a few CCSs per module like this which are never registered)
 - None of the dynamically allocated CCSs are registered.

 My question is: am I misunderstanding how the CCS registry (`CCS_LIST`) is
 supposed to be used? I thought all CCSs should really be in that list,
 because
 the list is used when reporting. Not registering a CCS means not reporting
 it.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16143
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler