rts: gc stats: account properly for copied bytes in sequential collections

We were not updating the [copied,any_work,scav_find_work, max_n_todo_overflow]
counters during sequential collections. As well, we were double counting for
parallel collections.

To fix this we add an `else` clause to the `if (is_par_gc())`.

The par_* counters do not need to be updated in the sequential case
because they must be 0.

(cherry picked from commit eb0431489144effd6931c248801af3fe65227368)

2 files changed, 10 insertions, 0 deletions

diff --git a/docs/users_guide/9.4.1-notes.rst b/docs/users_guide/9.4.1-notes.rst
index 8fd8374136..d24e55442a 100644
--- a/docs/users_guide/9.4.1-notes.rst
+++ b/docs/users_guide/9.4.1-notes.rst
@@ -222,6 +222,9 @@ Runtime system
   eliminating the need to pass the :ghc-flag:`-eventlog` flag to use the eventlog.
   This flag has been deprecated (:ghc-ticket:`18948`).
 
+- Summary statistics, i.e. the output of :rts-flag:`-s [⟨file⟩]`, now correctly
+  accounts for bytes copied during sequential collections.
+
 ``base`` library
 ~~~~~~~~~~~~~~~~
 
diff --git a/rts/sm/GC.c b/rts/sm/GC.c
index 15aef3a9fc..0116d0033f 100644
--- a/rts/sm/GC.c
+++ b/rts/sm/GC.c
@@ -170,6 +170,9 @@ is requested it takes 1, when a SYNC_GC_PAR is requested it takes n_capabilities
 
 Clearly this is in need of some tidying up, but for now we tread carefully. We
 call is_par_gc() to see whether we are in a parallel or sequential collection.
+If we are in a parallel collection we iterate over gc_threads, being careful to
+account for idle caps. If we are in a sequential collection we deal only with
+the thread local gct.
 Of course this is valid only inside GarbageCollect ().
 
 Omitting this check has led to issues such as #19147.
@@ -647,7 +650,11 @@ GarbageCollect (uint32_t collect_gen,
           par_balanced_copied =
               (par_balanced_copied_acc - copied + other_active_threads / 2) /
               other_active_threads;
+      } else {
           copied += gct->copied;
+          any_work += gct->any_work;
+          scav_find_work += gct->scav_find_work;
+          max_n_todo_overflow += gct->max_n_todo_overflow;
       }
   }