fix for excessive gen0 in high memory load situation (#61884)

in high memory load situations, when we had to wait for a BGC to finish, we switched to preemptive mode which means GCs could have occurred and replenished the budget. but when we come back from the wait, we use the previous decision on whether the budget was exceeded or not. this causes us to trigger GC incorrect and you would see GCs triggered when the allocated bytes are tiny.

Author: Maoni0

src/coreclr/gc/gc.cpp

@@ -7151,7 +7151,6 @@ void gc_mechanisms::init_mechanisms()
     found_finalizers = FALSE;
 #ifdef BACKGROUND_GC
     background_p = gc_heap::background_running_p() != FALSE;
-    allocations_allowed = TRUE;
 #endif //BACKGROUND_GC
 
     entry_memory_load = 0;
@@ -7444,15 +7443,6 @@ void gc_heap::reset_allocation_pointers (generation* gen, uint8_t* start)
 
 bool gc_heap::new_allocation_allowed (int gen_number)
 {
-#ifdef BACKGROUND_GC
-    //TODO BACKGROUND_GC this is for test only
-    if (!settings.allocations_allowed)
-    {
-        dprintf (2, ("new allocation not allowed"));
-        return FALSE;
-    }
-#endif //BACKGROUND_GC
-
     if (dd_new_allocation (dynamic_data_of (gen_number)) < 0)
     {
         if (gen_number != 0)
@@ -16084,18 +16074,22 @@ void gc_heap::wait_for_background (alloc_wait_reason awr, bool loh_p)
     add_saved_spinlock_info (loh_p, me_acquire, mt_wait_bgc);
 }
 
-void gc_heap::wait_for_bgc_high_memory (alloc_wait_reason awr, bool loh_p)
+bool gc_heap::wait_for_bgc_high_memory (alloc_wait_reason awr, bool loh_p)
 {
+    bool wait_p = false;
     if (gc_heap::background_running_p())
     {
         uint32_t memory_load;
         get_memory_info (&memory_load);
         if (memory_load >= m_high_memory_load_th)
         {
+            wait_p = true;
             dprintf (GTC_LOG, ("high mem - wait for BGC to finish, wait reason: %d", awr));
             wait_for_background (awr, loh_p);
         }
     }
+
+    return wait_p;
 }
 
 #endif //BACKGROUND_GC
@@ -17203,18 +17197,27 @@ allocation_state gc_heap::try_allocate_more_space (alloc_context* acontext, size
             }
 
 #ifdef BACKGROUND_GC
-            wait_for_bgc_high_memory (awr_gen0_alloc, loh_p);
+            bool recheck_p = wait_for_bgc_high_memory (awr_gen0_alloc, loh_p);
 #endif //BACKGROUND_GC
 
 #ifdef SYNCHRONIZATION_STATS
             bad_suspension++;
 #endif //SYNCHRONIZATION_STATS
             dprintf (2, ("h%d running out of budget on gen%d, gc", heap_number, gen_number));
 
-            if (!settings.concurrent || (gen_number == 0))
+#ifdef BACKGROUND_GC
+            bool trigger_gc_p = true;
+            if (recheck_p)
+                trigger_gc_p = !(new_allocation_allowed (gen_number));
+
+            if (trigger_gc_p)
+#endif //BACKGROUND_GC
             {
-                trigger_gc_for_alloc (0, ((gen_number == 0) ? reason_alloc_soh : reason_alloc_loh),
-                                    msl, loh_p, mt_try_budget);
+                if (!settings.concurrent || (gen_number == 0))
+                {
+                    trigger_gc_for_alloc (0, ((gen_number == 0) ? reason_alloc_soh : reason_alloc_loh),
+                                        msl, loh_p, mt_try_budget);
+                }
             }
         }
     }

src/coreclr/gc/gcpriv.h

@@ -504,7 +504,6 @@ class gc_mechanisms
 #ifdef BACKGROUND_GC
     BOOL background_p;
     bgc_state b_state;
-    BOOL allocations_allowed;
 #endif //BACKGROUND_GC
 
 #ifdef STRESS_HEAP
@@ -1791,7 +1790,7 @@ class gc_heap
     void wait_for_background (alloc_wait_reason awr, bool loh_p);
 
     PER_HEAP
-    void wait_for_bgc_high_memory (alloc_wait_reason awr, bool loh_p);
+    bool wait_for_bgc_high_memory (alloc_wait_reason awr, bool loh_p);
 
     PER_HEAP
     void bgc_uoh_alloc_clr (uint8_t* alloc_start,