From e48ce7ef0ef8de3d8e9e56be4d23d7830681b18b Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Mon, 3 May 2021 12:11:33 +0200
Subject: [PATCH] Prevent lwlock dtrace probes from unnecessary work

If dtrace is compiled in but disabled, the lwlock dtrace probes still
evaluate their arguments.  Since PostgreSQL 13, T_NAME(lock) does
nontrivial work, so it should be avoided if not needed.  To fix, make
these calls conditional on the *_ENABLED() macro corresponding to each
probe.

Reviewed-by: Craig Ringer <craig.ringer@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/CAGRY4nwxKUS_RvXFW-ugrZBYxPFFM5kjwKT5O+0+Stuga5b4+Q@mail.gmail.com
---
 src/backend/storage/lmgr/lwlock.c | 39 ++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 13 deletions(-)

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 2fa90cc095..b391f8924d 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -1323,7 +1323,8 @@ LWLockAcquire(LWLock *lock, LWLockMode mode)
 #endif
 
 		LWLockReportWaitStart(lock);
-		TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);
+		if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
+			TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);
 
 		for (;;)
 		{
@@ -1345,7 +1346,8 @@ LWLockAcquire(LWLock *lock, LWLockMode mode)
 		}
 #endif
 
-		TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
+		if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
+			TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
 		LWLockReportWaitEnd();
 
 		LOG_LWDEBUG("LWLockAcquire", lock, "awakened");
@@ -1354,7 +1356,8 @@ LWLockAcquire(LWLock *lock, LWLockMode mode)
 		result = false;
 	}
 
-	TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), mode);
+	if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_ENABLED())
+		TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), mode);
 
 	/* Add lock to list of locks held by this backend */
 	held_lwlocks[num_held_lwlocks].lock = lock;
@@ -1405,14 +1408,16 @@ LWLockConditionalAcquire(LWLock *lock, LWLockMode mode)
 		RESUME_INTERRUPTS();
 
 		LOG_LWDEBUG("LWLockConditionalAcquire", lock, "failed");
-		TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(T_NAME(lock), mode);
+		if (TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL_ENABLED())
+			TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(T_NAME(lock), mode);
 	}
 	else
 	{
 		/* Add lock to list of locks held by this backend */
 		held_lwlocks[num_held_lwlocks].lock = lock;
 		held_lwlocks[num_held_lwlocks++].mode = mode;
-		TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE(T_NAME(lock), mode);
+		if (TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_ENABLED())
+			TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE(T_NAME(lock), mode);
 	}
 	return !mustwait;
 }
@@ -1484,7 +1489,8 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
 #endif
 
 			LWLockReportWaitStart(lock);
-			TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);
+			if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
+				TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);
 
 			for (;;)
 			{
@@ -1502,7 +1508,8 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
 				Assert(nwaiters < MAX_BACKENDS);
 			}
 #endif
-			TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
+			if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
+				TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
 			LWLockReportWaitEnd();
 
 			LOG_LWDEBUG("LWLockAcquireOrWait", lock, "awakened");
@@ -1532,7 +1539,8 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
 		/* Failed to get lock, so release interrupt holdoff */
 		RESUME_INTERRUPTS();
 		LOG_LWDEBUG("LWLockAcquireOrWait", lock, "failed");
-		TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL(T_NAME(lock), mode);
+		if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL_ENABLED())
+			TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL(T_NAME(lock), mode);
 	}
 	else
 	{
@@ -1540,7 +1548,8 @@ LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
 		/* Add lock to list of locks held by this backend */
 		held_lwlocks[num_held_lwlocks].lock = lock;
 		held_lwlocks[num_held_lwlocks++].mode = mode;
-		TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT(T_NAME(lock), mode);
+		if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_ENABLED())
+			TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT(T_NAME(lock), mode);
 	}
 
 	return !mustwait;
@@ -1700,7 +1709,8 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
 #endif
 
 		LWLockReportWaitStart(lock);
-		TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), LW_EXCLUSIVE);
+		if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
+			TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), LW_EXCLUSIVE);
 
 		for (;;)
 		{
@@ -1719,7 +1729,8 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
 		}
 #endif
 
-		TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), LW_EXCLUSIVE);
+		if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
+			TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), LW_EXCLUSIVE);
 		LWLockReportWaitEnd();
 
 		LOG_LWDEBUG("LWLockWaitForVar", lock, "awakened");
@@ -1727,7 +1738,8 @@ LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
 		/* Now loop back and check the status of the lock again. */
 	}
 
-	TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), LW_EXCLUSIVE);
+	if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_ENABLED())
+		TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), LW_EXCLUSIVE);
 
 	/*
 	 * Fix the process wait semaphore's count for any absorbed wakeups.
@@ -1870,7 +1882,8 @@ LWLockRelease(LWLock *lock)
 		LWLockWakeup(lock);
 	}
 
-	TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock));
+	if (TRACE_POSTGRESQL_LWLOCK_RELEASE_ENABLED())
+		TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock));
 
 	/*
 	 * Now okay to allow cancel/die interrupts.