[compiler-rt] [llvm] [tsan] Introduce Adaptive Delay Scheduling to TSAN (PR #178836)

[Dmitry Vyukov via llvm-commits]

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+//===-- tsan_adaptive_delay.h -----------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is a part of ThreadSanitizer (TSan), a race detector.
+//
+//===----------------------------------------------------------------------===//
+
+#include "tsan_adaptive_delay.h"
+
+#include "interception/interception.h"
+#include "sanitizer_common/sanitizer_allocator_internal.h"
+#include "sanitizer_common/sanitizer_common.h"
+#include "sanitizer_common/sanitizer_errno_codes.h"
+#include "tsan_interface.h"
+#include "tsan_rtl.h"
+
+namespace __tsan {
+
+namespace {
+
+// =============================================================================
+// DelaySpec: Represents a delay configuration parsed from flag strings
+// =============================================================================
+//
+// Delay can be specified as:
+//   - "spin=N"     : Spin for up to N cycles (very short delays)
+//   - "yield"      : Call sched_yield() once
+//   - "sleep_us=N" : Sleep for up to N microseconds
+
+enum class DelayType { Spin, Yield, SleepUs };
+
+struct DelaySpec {
+  DelayType type;
+  int value;  // spin cycles or sleep_us value; ignored for yield
+
+  // Estimated nanoseconds per spin cycle (volatile loop iteration)
+  static constexpr u64 kNsPerSpinCycle = 5;
+  // Estimated nanoseconds for a yield (context switch overhead)
+  static constexpr u64 kNsPerYield = 500;
+
+  static DelaySpec Parse(const char* str) {
+    DelaySpec spec;
+    if (internal_strncmp(str, "spin=", 5) == 0) {
+      spec.type = DelayType::Spin;
+      spec.value = internal_atoll(str + 5);
+      if (spec.value <= 0)
+        spec.value = 10;
+    } else if (internal_strcmp(str, "yield") == 0) {
+      spec.type = DelayType::Yield;
+      spec.value = 0;
+    } else if (internal_strncmp(str, "sleep_us=", 9) == 0) {
+      spec.type = DelayType::SleepUs;
+      spec.value = internal_atoll(str + 9);
+      if (spec.value <= 0)
+        spec.value = 1;
+    } else {
+      // Default to yield if unrecognized
+      Printf("WARNING: Unrecognized delay spec '%s', defaulting to yield\n",
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dvyukov wrote:

We already fail on some incorrect values, let's try to hard fail here as well. Since it's a new option, it should be safe.

https://github.com/llvm/llvm-project/pull/178836