[compiler-rt] acbb4fc - tsan: increase max number of threads supported by test-only barrier

[Dmitry Vyukov via llvm-commits]

Author: Dmitry Vyukov
Date: 2021-07-28T17:35:23+02:00
New Revision: acbb4fcd5e664ddfb515785f1839afc8f2570035

URL: https://github.com/llvm/llvm-project/commit/acbb4fcd5e664ddfb515785f1839afc8f2570035
DIFF: https://github.com/llvm/llvm-project/commit/acbb4fcd5e664ddfb515785f1839afc8f2570035.diff

LOG: tsan: increase max number of threads supported by test-only barrier

Currently the barrier supports only 256 threads,
this does not allow to write reliable tests that use more threads.
Bump max number of threads to 1024 to support writing
good stress tests.
Also replace sched_yield() with usleep(100) on the wait path.
If we write tests that create hundreds of threads (and dozens
of tests can run in parallel), yield would consume massive
amounts of CPU time for spinning.

Depends on D106952.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D106953

Added: 
    

Modified: 
    compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp

Removed: 
    


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diff  --git a/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp b/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
index dd24428427952..f7f8edef8e785 100644
--- a/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
+++ b/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
@@ -2920,25 +2920,30 @@ void InitializeInterceptors() {
 // Note that no_sanitize_thread attribute does not turn off atomic interception
 // so attaching it to the function defined in user code does not help.
 // That's why we now have what we have.
+constexpr uptr kBarrierThreadBits = 10;
+constexpr uptr kBarrierThreads = 1 << kBarrierThreadBits;
+
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
 void __tsan_testonly_barrier_init(u64 *barrier, u32 count) {
-  if (count >= (1 << 8)) {
-      Printf("barrier_init: count is too large (%d)\n", count);
-      Die();
+  if (count >= kBarrierThreads) {
+    Printf("barrier_init: count is too large (%d)\n", count);
+    Die();
   }
-  // 8 lsb is thread count, the remaining are count of entered threads.
+  // kBarrierThreadBits lsb is thread count,
+  // the remaining are count of entered threads.
   *barrier = count;
 }
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
 void __tsan_testonly_barrier_wait(u64 *barrier) {
-  unsigned old = __atomic_fetch_add(barrier, 1 << 8, __ATOMIC_RELAXED);
-  unsigned old_epoch = (old >> 8) / (old & 0xff);
+  constexpr uptr kThreadMask = kBarrierThreads - 1;
+  unsigned old = __atomic_fetch_add(barrier, kBarrierThreads, __ATOMIC_RELAXED);
+  unsigned old_epoch = (old >> kBarrierThreadBits) / (old & kThreadMask);
   for (;;) {
     unsigned cur = __atomic_load_n(barrier, __ATOMIC_RELAXED);
-    unsigned cur_epoch = (cur >> 8) / (cur & 0xff);
+    unsigned cur_epoch = (cur >> kBarrierThreadBits) / (cur & kThreadMask);
     if (cur_epoch != old_epoch)
-      return;
-    internal_sched_yield();
+      break;
+    internal_usleep(100);
   }
 }