[Openmp-commits] [PATCH] D101882: [OpenMP] Fix hidden helper + affinity assignment

[Jonathan Peyton via Phabricator via Openmp-commits]

jlpeyton created this revision.
jlpeyton added reviewers: AndreyChurbanov, hbae, tlwilmar, jdoerfert.
jlpeyton added a project: OpenMP.
Herald added subscribers: guansong, yaxunl.
jlpeyton requested review of this revision.
Herald added a subscriber: sstefan1.

When `KMP_AFFINITY` is set, each thread's gtid value is used as an index into the place list to determine the thread's placement. With hidden helpers enabled, this gtid value is shifted down leading to unexpected shifted thread placement. This patch restores the previous behavior by adjusting the mask index to take the number of hidden helper threads into account.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D101882

Files:
  openmp/runtime/src/kmp.h
  openmp/runtime/src/kmp_affinity.cpp


Index: openmp/runtime/src/kmp_affinity.cpp
===================================================================
--- openmp/runtime/src/kmp_affinity.cpp
+++ openmp/runtime/src/kmp_affinity.cpp
@@ -3950,8 +3950,9 @@
       i = 0;
       mask = __kmp_affin_fullMask;
     } else {
+      int mask_idx = __kmp_adjust_gtid_for_hidden_helpers(gtid);
       KMP_DEBUG_ASSERT(__kmp_affinity_num_masks > 0);
-      i = (gtid + __kmp_affinity_offset) % __kmp_affinity_num_masks;
+      i = (mask_idx + __kmp_affinity_offset) % __kmp_affinity_num_masks;
       mask = KMP_CPU_INDEX(__kmp_affinity_masks, i);
     }
   } else {
@@ -3967,9 +3968,10 @@
       mask = __kmp_affin_fullMask;
     } else {
       // int i = some hash function or just a counter that doesn't
-      // always start at 0.  Use gtid for now.
+      // always start at 0.  Use adjusted gtid for now.
+      int mask_idx = __kmp_adjust_gtid_for_hidden_helpers(gtid);
       KMP_DEBUG_ASSERT(__kmp_affinity_num_masks > 0);
-      i = (gtid + __kmp_affinity_offset) % __kmp_affinity_num_masks;
+      i = (mask_idx + __kmp_affinity_offset) % __kmp_affinity_num_masks;
       mask = KMP_CPU_INDEX(__kmp_affinity_masks, i);
     }
   }
Index: openmp/runtime/src/kmp.h
===================================================================
--- openmp/runtime/src/kmp.h
+++ openmp/runtime/src/kmp.h
@@ -4072,6 +4072,18 @@
 #define KMP_GTID_TO_SHADOW_GTID(gtid)                                          \
   ((gtid) % (__kmp_hidden_helper_threads_num - 1) + 2)
 
+// Return the adjusted gtid value by subtracting from gtid the number
+// of hidden helper threads. This adjusted value is the gtid the thread would
+// have received if there were no hidden helper threads.
+static inline int __kmp_adjust_gtid_for_hidden_helpers(int gtid) {
+  int adjusted_gtid = gtid;
+  if (__kmp_enable_hidden_helper && __kmp_hidden_helper_threads_num > 0 &&
+      gtid > 0 && adjusted_gtid - __kmp_hidden_helper_threads_num >= 0) {
+    adjusted_gtid -= __kmp_hidden_helper_threads_num;
+  }
+  return adjusted_gtid;
+}
+
 // Support for error directive
 typedef enum kmp_severity_t {
   severity_warning = 1,


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D101882.342936.patch
Type: text/x-patch
Size: 2147 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/openmp-commits/attachments/20210505/0a5b1647/attachment.bin>