Do not recurse in the gravity pairs if the active cell is foreign.

src/runner_doiact_grav.h

@@ -935,6 +935,7 @@ void runner_dopair_grav(struct runner *r, struct cell *ci, struct cell *cj,
   /* Some constants */
   const struct engine *e = r->e;
   const struct space *s = e->s;
+  const int nodeID = e->nodeID;
   const int periodic = s->periodic;
   const double cell_width = s->width[0];
   const double dim[3] = {s->dim[0], s->dim[1], s->dim[2]};
@@ -943,6 +944,11 @@ void runner_dopair_grav(struct runner *r, struct cell *ci, struct cell *cj,
   const double max_distance = props->a_smooth * props->r_cut_max * cell_width;
   const double max_distance2 = max_distance * max_distance;
 
+  /* Anything to do here? */
+  if(!((cell_is_active(ci,e) && ci->nodeID == nodeID) ||
+       (cell_is_active(cj,e) && cj->nodeID == nodeID)))
+    return;
+
 #ifdef SWIFT_DEBUG_CHECKS
 
   const int gcount_i = ci->gcount;
@@ -963,9 +969,6 @@ void runner_dopair_grav(struct runner *r, struct cell *ci, struct cell *cj,
 
   TIMER_TIC;
 
-  /* Anything to do here? */
-  if (!cell_is_active(ci, e) && !cell_is_active(cj, e)) return;
-
   /* Recover the multipole information */
   struct gravity_tensors *const multi_i = ci->multipole;
   struct gravity_tensors *const multi_j = cj->multipole;