fix: set wallprecheck default flag to false

Author: kaahos

ddprof-lib/src/main/cpp/arguments.h

@@ -205,7 +205,7 @@ class Arguments {
         _cpu(-1),
         _wall(-1),
         _wall_collapsing(false),
-        _wall_precheck(true),
+        _wall_precheck(false),
         _wall_threads_per_tick(DEFAULT_WALL_THREADS_PER_TICK),
         _wallclock_sampler(ASGCT),
         _memory(-1),

ddprof-test/src/test/java/com/datadoghq/profiler/wallclock/PrecheckEfficiencyTest.java

@@ -25,10 +25,11 @@
  * wallprecheck=false} so every state is sampled; percentages estimate how often {@code
  * wallprecheck=true} would skip {@code SIGVTALRM} based on HotSpot {@code OSThreadState}.
  *
- * <p><strong>OS-state precheck (default {@code wallprecheck=true})</strong> skips signals when the
- * JVM reports {@code SLEEPING} or {@code CONDVAR_WAIT}. That covers Thread.sleep (legacy {@code
- * SLEEPING}; JDK 21+ sleep uses wait/park), {@code LockSupport.park}, and other condvar waits — not
- * {@code Object.wait} ({@code OBJECT_WAIT}), which the current precheck does not skip.
+ * <p><strong>OS-state precheck (opt-in via {@code wallprecheck=true}; default is {@code false})</strong>
+ * skips signals when the JVM reports {@code SLEEPING} or {@code CONDVAR_WAIT}. That covers
+ * Thread.sleep (legacy {@code SLEEPING}; JDK 21+ sleep uses wait/park), {@code LockSupport.park}, and
+ * other condvar waits — not {@code Object.wait} ({@code OBJECT_WAIT}), which the current precheck
+ * does not skip.
  *
  * <p>Sample classification prefers Java thread name ({@code EVENT_THREAD_NAME}) for the fixed worker
  * threads when present, then JFR thread state, then stack strings — some JVMs (e.g. Graal on aarch64)

ddprof-test/src/test/java/com/datadoghq/profiler/wallclock/PrecheckTest.java

@@ -11,7 +11,7 @@
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 /**
- * Verifies OS-thread-state precheck ({@code wallprecheck=true}, the default): the wall-clock timer
+ * Verifies OS-thread-state precheck (opt-in via {@code wallprecheck=true}): the wall-clock timer
  * thread reads {@code VMThread::osThreadState()} before {@code SIGVTALRM} and skips sending the
  * signal when HotSpot reports {@code SLEEPING} or {@code CONDVAR_WAIT}. Legacy JDKs report {@code
  * Thread.sleep} as {@code SLEEPING}; JDK 21+ uses a condvar wait path for sleep (often {@code
@@ -59,6 +59,6 @@ public void testSleepingThreadIsNotSampled() throws InterruptedException {
 
     @Override
     protected String getProfilerCommand() {
-        return "wall=1ms";  // wallprecheck=true by default
+        return "wall=1ms,wallprecheck=true";
     }
 }

ddprof-test/src/test/java/com/datadoghq/profiler/wallclock/WallclockMitigationsCombinedTest.java

@@ -116,9 +116,10 @@ public void precheckAndParkSuppressionWorkTogether() throws Exception {
         }
     }
 
-    /** Enables wall-clock profiling with default precheck behavior for combined assertions. */
+    /** Enables wall-clock profiling with the OS-state precheck explicitly turned on so both
+     * Approach A (precheck) and Approach B (park flag) are exercised in this combined test. */
     @Override
     protected String getProfilerCommand() {
-        return "wall=1ms,filter=0";
+        return "wall=1ms,filter=0,wallprecheck=true";
     }
 }

doc/architecture/TLSContext.md

@@ -508,12 +508,13 @@ Full benchmark data and analysis:
 snapshots for wall-clock collapsing and TaskBlock emission. The parked flag suppresses
 wall-clock signals on filtered threads when enabled.
 
-### `wallprecheck` (default `true`)
+### `wallprecheck` (default `false`, opt-in)
 
-When thread filtering is enabled and `Arguments::_wall_precheck` is true, the wall-clock
-timer samples **HotSpot** `VMThread::osThreadState()` **before** sending `SIGVTALRM`.
-Slots must hold a non-null `VMThread*` from registration (`ThreadFilter::setVMThread`)
-for the precheck to run; otherwise only Java park-flag suppression applies.
+Disabled by default. When explicitly enabled with `wallprecheck=true` *and* thread filtering
+is on (`Arguments::_wall_precheck == true`), the wall-clock timer samples **HotSpot**
+`VMThread::osThreadState()` **before** sending `SIGVTALRM`. Slots must hold a non-null
+`VMThread*` from registration (`ThreadFilter::setVMThread`) for the precheck to run;
+otherwise only Java park-flag suppression applies.
 
 On JDK **21+**, `Thread.sleep` often maps to **CONDVAR_WAIT** (not only legacy **SLEEPING**),
 same broad category as condvar-based park — see `PrecheckTest` and `wallClock.cpp`.
@@ -524,8 +525,10 @@ JVMTI `MonitorWait` / `MonitorWaited` callbacks are registered only when
 `java_version() < 21` (`vmEntry.cpp`). They populate TaskBlock with a **blocker** value
 matching **`System.identityHashCode(waitedObject)`** (same convention as the JNI park path).
 
-For **`wallprecheck=false`** (isolation tests such as `ParkTaskBlockTest`), OS-thread-state
-suppression is disabled so parked-flag behavior can be tested without condvar/sleep overlap.
+With the default **`wallprecheck=false`**, OS-thread-state suppression is off, so parked-flag
+behavior can be tested without condvar/sleep overlap (isolation tests such as
+`ParkTaskBlockTest`). The precheck path is exercised by `PrecheckTest` and
+`WallclockMitigationsCombinedTest`, which both pass `wallprecheck=true` explicitly.
 
 ## OTEP References