src/backend/utils/mmgr/redzone_handler.c - hawq - Git at Google

 /*
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements.  See the NOTICE file
  * distributed with this work for additional information
  * regarding copyright ownership.  The ASF licenses this file
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
  *
  *   http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing,
  * software distributed under the License is distributed on an
  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  * KIND, either express or implied.  See the License for the
  * specific language governing permissions and limitations
  * under the License.
  */

 /*-------------------------------------------------------------------------
  *
  * redzone_handler.c
  *	 Implementation of the red-zone handler that detects when the system
  *	 is running low in vmem (i.e., the system is in red-zone). The red-zone
  *	 handler identifies the session that consumes most vmem and asks it
  *	 to gracefully release its memory.
  *
  *-------------------------------------------------------------------------
  */

 #include "postgres.h"
 #include "utils/atomic.h"
 #include "cdb/cdbvars.h"
 #include "miscadmin.h"
 #include "utils/vmem_tracker.h"
 #include "utils/session_state.h"

 /* External dependencies within the runaway cleanup framework */
 extern bool vmemTrackerInited;
 extern volatile int32 *segmentVmemChunks;
 extern volatile int32 *segmentVmemQuotaChunks;
 extern volatile EventVersion *CurrentVersion;
 extern volatile EventVersion *latestRunawayVersion;
 extern void RunawayCleaner_StartCleanup(void);
 extern int32 VmemTracker_ConvertVmemMBToChunks(int mb);

 #define SHMEM_RUNAWAY_DETECTOR_MUTEX "SHMEM_RUNAWAY_DETECTOR_MUTEX"
 #define INVALID_SESSION_ID -1

 /* The runaway detector activates if the used vmem exceeds this percentage of the vmem quota */
 int	runaway_detector_activation_percent = 95;

 /* The last number of chunks for segment vmem quota */
 static int lastSegmentVmemQuotaChunks = -1;

 /* The number of memory chunks for red zone */
 static int redZoneChunks = -1;

 /*
  * A shared memory binary flag (0 or 1) that identifies one process at-a-time as runaway
  * detector. At red-zone each process tries to determine runaway query, but only the first
  * process that succeeds to set this counter to 1 becomes the detector.
  */
 volatile uint32 *isRunawayDetector = NULL;

 void RedZoneHandler_ShmemInit(void);
 void RedZoneHandler_ReactivateRunawayDetector(void);

 /*
  * Returns the red-zone cut-off in "chunks" unit
  */
 int32
 RedZoneHandler_GetRedZoneLimitChunks()
 {
 	/*
 	 * runaway_detector_activation_percent = 100% is reserved for not enforcing runaway
 	 * detection by setting the redZoneChunks to an artificially high value. Also, during
 	 * gpinitsystem we may start a QD without initializing the hawq_re_memory_overcommit_max.
 	 * This may result in 0 vmem protect limit. In such case, we ensure that the
 	 * redZoneChunks is set to a large value.
 	 */
 	if (runaway_detector_activation_percent != 100)
 	{
 		/*
 		 * Calculate red zone threshold in MB, and then convert MB to "chunks"
 		 * using chunk size for efficient comparison to detect red zone
 		 */
 		if (lastSegmentVmemQuotaChunks != *segmentVmemQuotaChunks)
 		{
 			lastSegmentVmemQuotaChunks = *segmentVmemQuotaChunks;
 			redZoneChunks = (int)(lastSegmentVmemQuotaChunks *
                                   ((float) runaway_detector_activation_percent) /
                                   ((float) 100));
 		}
 	}

 	/* 0 means disable red-zone completely */
 	if (redZoneChunks == 0)
 	{
 		redZoneChunks = INT32_MAX;
 	}

 	return redZoneChunks;
 }

 /*
  * Returns the red-zone cut-off in "MB" unit
  */
 int32
 RedZoneHandler_GetRedZoneLimitMB()
 {
 	return VmemTracker_ConvertVmemChunksToMB(redZoneChunks);
 }

 /*
  * Initializes the red zone handler's shared memory states.
  */
 void
 RedZoneHandler_ShmemInit()
 {
 	Assert(!vmemTrackerInited);

 	bool		alreadyInShmem = false;

 	isRunawayDetector = (uint32 *)
 								ShmemInitStruct(SHMEM_RUNAWAY_DETECTOR_MUTEX,
 										sizeof(int32),
 										&alreadyInShmem);
 	Assert(alreadyInShmem || !IsUnderPostmaster);

 	Assert(NULL != isRunawayDetector);

 	if(!IsUnderPostmaster)
 	{
 		*isRunawayDetector = 0;
 	}
 }

 /*
  * Returns true if the system is in red-zone (too little VMEM)
  */
 bool
 RedZoneHandler_IsVmemRedZone()
 {
 	if (!vmemTrackerInited)
 	{
 		return false;
 	}

 	int32 redZoneChunks = RedZoneHandler_GetRedZoneLimitChunks();
 	Assert(redZoneChunks > 0);
 	return *segmentVmemChunks > redZoneChunks;
 }

 /*
  * Finds and notifies the top vmem consuming session.
  */
 static void
 RedZoneHandler_FlagTopConsumer()
 {
 	if (!vmemTrackerInited)
 	{
 		return;
 	}

 	Assert(NULL != MySessionState);

 	bool success = compare_and_swap_32((uint32*) isRunawayDetector, 0, 1);

 	/* If successful then this process must be the runaway detector */
 	AssertImply(success, 1 == *isRunawayDetector);

 	/*
 	 * Someone already determined the runaway query, so nothing to do. This
 	 * will also prevent re-entry to this method by a cleaning session.
 	 */
 	if (!success)
 	{
 		return;
 	}

 	/*
 	 * Grabbing a shared lock prevents others to modify the SessionState
 	 * data structure, therefore ensuring that we don't flag someone
 	 * who was already dying. A shared lock is enough as we access the
 	 * data structure in a read-only manner.
 	 */
 	LWLockAcquire(SessionStateLock, LW_SHARED);

 	int32 maxVmem = 0;
 	int32 maxActiveVmem = 0;
 	SessionState *maxActiveVmemSessionState = NULL;
 	SessionState *maxVmemSessionState = NULL;

 	SessionState *curSessionState = AllSessionStateEntries->usedList;

 	while (curSessionState != NULL)
 	{
 		Assert(INVALID_SESSION_ID != curSessionState->sessionId);

 		int32 curVmem = curSessionState->sessionVmem;

 		Assert(maxActiveVmem <= maxVmem);

 		if (curVmem > maxActiveVmem)
 		{
 			if (curVmem > maxVmem)
 			{
 				maxVmemSessionState = curSessionState;
 				maxVmem = curVmem;
 			}

 			/*
 			 * Only consider sessions with at least 1 active process. As we
 			 * are *not* grabbings locks, this does not guarantee that by the
 			 * time we finish walking all sessions the chosen session will
 			 * still have active process.
 			 */
 			if  (curSessionState->activeProcessCount > 0)
 			{
 				maxActiveVmemSessionState = curSessionState;
 				maxActiveVmem = curVmem;
 			}
 		}

 		curSessionState = curSessionState->next;
 	}

 	if (NULL != maxActiveVmemSessionState)
 	{
 		SpinLockAcquire(&maxActiveVmemSessionState->spinLock);

 		/*
 		 * Now that we grabbed lock, make sure we have at least 1 active process
 		 * before flagging this session for termination
 		 */
 		if (0 < maxActiveVmemSessionState->activeProcessCount)
 		{
 			/*
 			 * First update the runaway event detection version so that
 			 * an active process of the runaway session is forced to clean up before
 			 * it deactivates. As we grabbed the spin lock, no process of the runaway
 			 * session can deactivate unless we release the lock. The other sessions
 			 * don't care what global runaway version they observe as the runaway
 			 * event is not pertinent to them.
 			 *
 			 * We don't need any lock here as the runaway detector is singleton,
 			 * and only the detector can update this variable.
 			 */
 			*latestRunawayVersion = *CurrentVersion + 1;
 			/*
 			 * Make sure that the runaway event version is not shared with any other
 			 * processes, and not shared with any other deactivation/reactivation version
 			 */
 			*CurrentVersion = *CurrentVersion + 2;

 			Assert(CLEANUP_COUNTDOWN_BEFORE_RUNAWAY == maxActiveVmemSessionState->cleanupCountdown);
 			/*
 			 * Determine how many processes need to cleanup to mark the session clean.
 			 */
 			maxActiveVmemSessionState->cleanupCountdown = maxActiveVmemSessionState->activeProcessCount;

 			if (maxActiveVmemSessionState == maxVmemSessionState)
 			{
 				/* Finally signal the runaway process for cleanup */
 				maxActiveVmemSessionState->runawayStatus = RunawayStatus_PrimaryRunawaySession;
 			}
 			else
 			{
 				maxActiveVmemSessionState->runawayStatus = RunawayStatus_SecondaryRunawaySession;
 			}

 			/* Save the amount of vmem session was holding when it was flagged as runaway */
 			maxActiveVmemSessionState->sessionVmemRunaway = maxActiveVmemSessionState->sessionVmem;

 			/* Save the command count currently running in the runaway session */
 			maxActiveVmemSessionState->commandCountRunaway = gp_command_count;
 		}
 		else
 		{
 			/*
 			 * Failed to find any viable runaway session. Reset runaway detector flag
 			 * for another round of runaway determination at a later time. As we couldn't
 			 * find any runaway session, the CurrentVersion is not changed.
 			 */
 			*isRunawayDetector = 0;
 		}

 		SpinLockRelease(&maxActiveVmemSessionState->spinLock);
 	}
 	else
 	{
 		/*
 		 * No active session to mark as runaway. So, reenable the runaway detection process
 		 */
 		*isRunawayDetector = 0;
 	}

 	LWLockRelease(SessionStateLock);
 }

 /*
  * In a red-zone this method identifies the top vmem consuming session,
  * and requests it to cleanup. If the red-zone handler determines itself
  * as the runaway session, it also starts the cleanup.
  */
 void
 RedZoneHandler_DetectRunawaySession()
 {
 	/*
 	 * InterruptHoldoffCount > 0 indicates we are in a sensitive code path that doesn't
 	 * like a control flow disruption as may happen from a pending die/cancel interrupt.
 	 * As we may eventually ERROR out from this method (during RunawayCleaner_StartCleanup)
 	 * we want to make sure that HOLD_INTERRUPTS() was not called (i.e., InterruptHoldoffCount == 0).
 	 *
 	 * What happens if we don't check for InterruptHoldoffCount? One example is LWLockAcquire()
 	 * which calls HOLD_INTERRUPTS() to ensure that no unexpected control
 	 * flow disruption happens because of FATAL/ERROR as done from die/cancel interrupt
 	 * handler. If we ignore InterruptHoldoffCount, the PGSemaphoreLock() (called from LWLockAcquire)
 	 * would call CHECK_FOR_INTERRUPTS() and we may throw ERROR if the current session is a runaway.
 	 * Unfortunately, LWLockAcquire shares the semaphore with the regular lock manager and
 	 * ProcWaitForSignal. Therefore, LWLockAcquire may wake up multiple times during its wait
 	 * for a semaphore which may not relate to an actual LWLock release. This requires LWLockAcquire
 	 * to keep track of how many of those false wake events it has consumed (by decrementing semaphore
 	 * when it shouldn't have done so) and LWLockAcquire rollback the semaphore decrements for
 	 * the irrelevant wake up events by re-incrementing once it actually acquires the lock.
 	 * Therefore, an unexpected control flow out of the LWLockAcquire before it properly rolled back
 	 * may prevent the LWLockAcquire to rollback the false wake events. Although we do call LWLockRelease
 	 * during an error handling, that doesn't guarantee that the falsely consumed semaphore wake
 	 * events would be rolled back (i.e., semaphore does not get re-incremented during error handling) as
 	 * done at the end of LWLockAcquire. This may cause the semaphore to never wake up other waiting
 	 * processes and therefore may cause other processes to hang perpetually.
 	 */
 	if (!RedZoneHandler_IsVmemRedZone() || InterruptHoldoffCount > 0 ||
 			CritSectionCount > 0)
 	{
 		return;
 	}

 	/* We don't support runaway detection/termination from non-owner thread */
 	Assert(MemoryProtection_IsOwnerThread());
 	Assert(gp_mp_inited);

 	RedZoneHandler_FlagTopConsumer();
 	RunawayCleaner_StartCleanup();
 }

 /*
  * Saves VMEM usage of all the sessions into log
  */
 void
 RedZoneHandler_LogVmemUsageOfAllSessions()
 {
 	if (!vmemTrackerInited)
 	{
 		return;
 	}

 	Assert(NULL != MySessionState);

 	/*
 	 * Grabbing a shared lock ensures that the data structure is not
 	 * modified while we are reading. Shared lock is enough as we
 	 * are only reading and not modifying the SessionState data structure
 	 */
 	LWLockAcquire(SessionStateLock, LW_SHARED);

 	SessionState *curSessionState = AllSessionStateEntries->usedList;

 	PG_TRY();
 	{
 		/* Write the header for the subsequent lines of memory usage information */
 		write_stderr("session_state: session_id, is_runaway, qe_count, active_qe_count, dirty_qe_count, vmem_mb, runaway_vmem_mb, runaway_command_cnt\n");

 		while (curSessionState != NULL)
 		{
 			Assert(INVALID_SESSION_ID != curSessionState->sessionId);

 			write_stderr("session_state: %d, %d, %d, %d, %d, %d, %d, %d\n", curSessionState->sessionId,
 					curSessionState->runawayStatus != RunawayStatus_NotRunaway, curSessionState->pinCount,
 					curSessionState->activeProcessCount, curSessionState->cleanupCountdown, VmemTracker_ConvertVmemChunksToMB(curSessionState->sessionVmem),
 					VmemTracker_ConvertVmemChunksToMB(curSessionState->sessionVmemRunaway), curSessionState->commandCountRunaway);

 			curSessionState = curSessionState->next;
 		}
 	}
 	PG_CATCH();
 	{
 		LWLockRelease(SessionStateLock);
 		PG_RE_THROW();
 	}
 	PG_END_TRY();

 	LWLockRelease(SessionStateLock);
 }
	/*
	* Licensed to the Apache Software Foundation (ASF) under one
	* or more contributor license agreements. See the NOTICE file
	* distributed with this work for additional information
	* regarding copyright ownership. The ASF licenses this file
	* to you under the Apache License, Version 2.0 (the
	* "License"); you may not use this file except in compliance
	* with the License. You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing,
	* software distributed under the License is distributed on an
	* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
	* KIND, either express or implied. See the License for the
	* specific language governing permissions and limitations
	* under the License.
	*/

	/*-------------------------------------------------------------------------
	*
	* redzone_handler.c
	* Implementation of the red-zone handler that detects when the system
	* is running low in vmem (i.e., the system is in red-zone). The red-zone
	* handler identifies the session that consumes most vmem and asks it
	* to gracefully release its memory.
	*
	*-------------------------------------------------------------------------
	*/

	#include "postgres.h"
	#include "utils/atomic.h"
	#include "cdb/cdbvars.h"
	#include "miscadmin.h"
	#include "utils/vmem_tracker.h"
	#include "utils/session_state.h"

	/* External dependencies within the runaway cleanup framework */
	extern bool vmemTrackerInited;
	extern volatile int32 *segmentVmemChunks;
	extern volatile int32 *segmentVmemQuotaChunks;
	extern volatile EventVersion *CurrentVersion;
	extern volatile EventVersion *latestRunawayVersion;
	extern void RunawayCleaner_StartCleanup(void);
	extern int32 VmemTracker_ConvertVmemMBToChunks(int mb);

	#define SHMEM_RUNAWAY_DETECTOR_MUTEX "SHMEM_RUNAWAY_DETECTOR_MUTEX"
	#define INVALID_SESSION_ID -1

	/* The runaway detector activates if the used vmem exceeds this percentage of the vmem quota */
	int runaway_detector_activation_percent = 95;

	/* The last number of chunks for segment vmem quota */
	static int lastSegmentVmemQuotaChunks = -1;

	/* The number of memory chunks for red zone */
	static int redZoneChunks = -1;

	/*
	* A shared memory binary flag (0 or 1) that identifies one process at-a-time as runaway
	* detector. At red-zone each process tries to determine runaway query, but only the first
	* process that succeeds to set this counter to 1 becomes the detector.
	*/
	volatile uint32 *isRunawayDetector = NULL;

	void RedZoneHandler_ShmemInit(void);
	void RedZoneHandler_ReactivateRunawayDetector(void);

	/*
	* Returns the red-zone cut-off in "chunks" unit
	*/
	int32
	RedZoneHandler_GetRedZoneLimitChunks()
	{
	/*
	* runaway_detector_activation_percent = 100% is reserved for not enforcing runaway
	* detection by setting the redZoneChunks to an artificially high value. Also, during
	* gpinitsystem we may start a QD without initializing the hawq_re_memory_overcommit_max.
	* This may result in 0 vmem protect limit. In such case, we ensure that the
	* redZoneChunks is set to a large value.
	*/
	if (runaway_detector_activation_percent != 100)
	{
	/*
	* Calculate red zone threshold in MB, and then convert MB to "chunks"
	* using chunk size for efficient comparison to detect red zone
	*/
	if (lastSegmentVmemQuotaChunks != *segmentVmemQuotaChunks)
	{
	lastSegmentVmemQuotaChunks = *segmentVmemQuotaChunks;
	redZoneChunks = (int)(lastSegmentVmemQuotaChunks *
	((float) runaway_detector_activation_percent) /
	((float) 100));
	}
	}

	/* 0 means disable red-zone completely */
	if (redZoneChunks == 0)
	{
	redZoneChunks = INT32_MAX;
	}

	return redZoneChunks;
	}

	/*
	* Returns the red-zone cut-off in "MB" unit
	*/
	int32
	RedZoneHandler_GetRedZoneLimitMB()
	{
	return VmemTracker_ConvertVmemChunksToMB(redZoneChunks);
	}

	/*
	* Initializes the red zone handler's shared memory states.
	*/
	void
	RedZoneHandler_ShmemInit()
	{
	Assert(!vmemTrackerInited);

	bool alreadyInShmem = false;

	isRunawayDetector = (uint32 *)
	ShmemInitStruct(SHMEM_RUNAWAY_DETECTOR_MUTEX,
	sizeof(int32),
	&alreadyInShmem);
	Assert(alreadyInShmem \|\| !IsUnderPostmaster);

	Assert(NULL != isRunawayDetector);

	if(!IsUnderPostmaster)
	{
	*isRunawayDetector = 0;
	}
	}

	/*
	* Returns true if the system is in red-zone (too little VMEM)
	*/
	bool
	RedZoneHandler_IsVmemRedZone()
	{
	if (!vmemTrackerInited)
	{
	return false;
	}

	int32 redZoneChunks = RedZoneHandler_GetRedZoneLimitChunks();
	Assert(redZoneChunks > 0);
	return *segmentVmemChunks > redZoneChunks;
	}

	/*
	* Finds and notifies the top vmem consuming session.
	*/
	static void
	RedZoneHandler_FlagTopConsumer()
	{
	if (!vmemTrackerInited)
	{
	return;
	}

	Assert(NULL != MySessionState);

	bool success = compare_and_swap_32((uint32*) isRunawayDetector, 0, 1);

	/* If successful then this process must be the runaway detector */
	AssertImply(success, 1 == *isRunawayDetector);

	/*
	* Someone already determined the runaway query, so nothing to do. This
	* will also prevent re-entry to this method by a cleaning session.
	*/
	if (!success)
	{
	return;
	}

	/*
	* Grabbing a shared lock prevents others to modify the SessionState
	* data structure, therefore ensuring that we don't flag someone
	* who was already dying. A shared lock is enough as we access the
	* data structure in a read-only manner.
	*/
	LWLockAcquire(SessionStateLock, LW_SHARED);

	int32 maxVmem = 0;
	int32 maxActiveVmem = 0;
	SessionState *maxActiveVmemSessionState = NULL;
	SessionState *maxVmemSessionState = NULL;

	SessionState *curSessionState = AllSessionStateEntries->usedList;

	while (curSessionState != NULL)
	{
	Assert(INVALID_SESSION_ID != curSessionState->sessionId);

	int32 curVmem = curSessionState->sessionVmem;

	Assert(maxActiveVmem <= maxVmem);

	if (curVmem > maxActiveVmem)
	{
	if (curVmem > maxVmem)
	{
	maxVmemSessionState = curSessionState;
	maxVmem = curVmem;
	}

	/*
	* Only consider sessions with at least 1 active process. As we
	* are not grabbings locks, this does not guarantee that by the
	* time we finish walking all sessions the chosen session will
	* still have active process.
	*/
	if (curSessionState->activeProcessCount > 0)
	{
	maxActiveVmemSessionState = curSessionState;
	maxActiveVmem = curVmem;
	}
	}

	curSessionState = curSessionState->next;
	}

	if (NULL != maxActiveVmemSessionState)
	{
	SpinLockAcquire(&maxActiveVmemSessionState->spinLock);

	/*
	* Now that we grabbed lock, make sure we have at least 1 active process
	* before flagging this session for termination
	*/
	if (0 < maxActiveVmemSessionState->activeProcessCount)
	{
	/*
	* First update the runaway event detection version so that
	* an active process of the runaway session is forced to clean up before
	* it deactivates. As we grabbed the spin lock, no process of the runaway
	* session can deactivate unless we release the lock. The other sessions
	* don't care what global runaway version they observe as the runaway
	* event is not pertinent to them.
	*
	* We don't need any lock here as the runaway detector is singleton,
	* and only the detector can update this variable.
	*/
	latestRunawayVersion = CurrentVersion + 1;
	/*
	* Make sure that the runaway event version is not shared with any other
	* processes, and not shared with any other deactivation/reactivation version
	*/
	CurrentVersion = CurrentVersion + 2;

	Assert(CLEANUP_COUNTDOWN_BEFORE_RUNAWAY == maxActiveVmemSessionState->cleanupCountdown);
	/*
	* Determine how many processes need to cleanup to mark the session clean.
	*/
	maxActiveVmemSessionState->cleanupCountdown = maxActiveVmemSessionState->activeProcessCount;

	if (maxActiveVmemSessionState == maxVmemSessionState)
	{
	/* Finally signal the runaway process for cleanup */
	maxActiveVmemSessionState->runawayStatus = RunawayStatus_PrimaryRunawaySession;
	}
	else
	{
	maxActiveVmemSessionState->runawayStatus = RunawayStatus_SecondaryRunawaySession;
	}

	/* Save the amount of vmem session was holding when it was flagged as runaway */
	maxActiveVmemSessionState->sessionVmemRunaway = maxActiveVmemSessionState->sessionVmem;

	/* Save the command count currently running in the runaway session */
	maxActiveVmemSessionState->commandCountRunaway = gp_command_count;
	}
	else
	{
	/*
	* Failed to find any viable runaway session. Reset runaway detector flag
	* for another round of runaway determination at a later time. As we couldn't
	* find any runaway session, the CurrentVersion is not changed.
	*/
	*isRunawayDetector = 0;
	}

	SpinLockRelease(&maxActiveVmemSessionState->spinLock);
	}
	else
	{
	/*
	* No active session to mark as runaway. So, reenable the runaway detection process
	*/
	*isRunawayDetector = 0;
	}

	LWLockRelease(SessionStateLock);
	}

	/*
	* In a red-zone this method identifies the top vmem consuming session,
	* and requests it to cleanup. If the red-zone handler determines itself
	* as the runaway session, it also starts the cleanup.
	*/
	void
	RedZoneHandler_DetectRunawaySession()
	{
	/*
	* InterruptHoldoffCount > 0 indicates we are in a sensitive code path that doesn't
	* like a control flow disruption as may happen from a pending die/cancel interrupt.
	* As we may eventually ERROR out from this method (during RunawayCleaner_StartCleanup)
	* we want to make sure that HOLD_INTERRUPTS() was not called (i.e., InterruptHoldoffCount == 0).
	*
	* What happens if we don't check for InterruptHoldoffCount? One example is LWLockAcquire()
	* which calls HOLD_INTERRUPTS() to ensure that no unexpected control
	* flow disruption happens because of FATAL/ERROR as done from die/cancel interrupt
	* handler. If we ignore InterruptHoldoffCount, the PGSemaphoreLock() (called from LWLockAcquire)
	* would call CHECK_FOR_INTERRUPTS() and we may throw ERROR if the current session is a runaway.
	* Unfortunately, LWLockAcquire shares the semaphore with the regular lock manager and
	* ProcWaitForSignal. Therefore, LWLockAcquire may wake up multiple times during its wait
	* for a semaphore which may not relate to an actual LWLock release. This requires LWLockAcquire
	* to keep track of how many of those false wake events it has consumed (by decrementing semaphore
	* when it shouldn't have done so) and LWLockAcquire rollback the semaphore decrements for
	* the irrelevant wake up events by re-incrementing once it actually acquires the lock.
	* Therefore, an unexpected control flow out of the LWLockAcquire before it properly rolled back
	* may prevent the LWLockAcquire to rollback the false wake events. Although we do call LWLockRelease
	* during an error handling, that doesn't guarantee that the falsely consumed semaphore wake
	* events would be rolled back (i.e., semaphore does not get re-incremented during error handling) as
	* done at the end of LWLockAcquire. This may cause the semaphore to never wake up other waiting
	* processes and therefore may cause other processes to hang perpetually.
	*/
	if (!RedZoneHandler_IsVmemRedZone() \|\| InterruptHoldoffCount > 0 \|\|
	CritSectionCount > 0)
	{
	return;
	}

	/* We don't support runaway detection/termination from non-owner thread */
	Assert(MemoryProtection_IsOwnerThread());
	Assert(gp_mp_inited);

	RedZoneHandler_FlagTopConsumer();
	RunawayCleaner_StartCleanup();
	}

	/*
	* Saves VMEM usage of all the sessions into log
	*/
	void
	RedZoneHandler_LogVmemUsageOfAllSessions()
	{
	if (!vmemTrackerInited)
	{
	return;
	}

	Assert(NULL != MySessionState);

	/*
	* Grabbing a shared lock ensures that the data structure is not
	* modified while we are reading. Shared lock is enough as we
	* are only reading and not modifying the SessionState data structure
	*/
	LWLockAcquire(SessionStateLock, LW_SHARED);

	SessionState *curSessionState = AllSessionStateEntries->usedList;

	PG_TRY();
	{
	/* Write the header for the subsequent lines of memory usage information */
	write_stderr("session_state: session_id, is_runaway, qe_count, active_qe_count, dirty_qe_count, vmem_mb, runaway_vmem_mb, runaway_command_cnt\n");

	while (curSessionState != NULL)
	{
	Assert(INVALID_SESSION_ID != curSessionState->sessionId);

	write_stderr("session_state: %d, %d, %d, %d, %d, %d, %d, %d\n", curSessionState->sessionId,
	curSessionState->runawayStatus != RunawayStatus_NotRunaway, curSessionState->pinCount,
	curSessionState->activeProcessCount, curSessionState->cleanupCountdown, VmemTracker_ConvertVmemChunksToMB(curSessionState->sessionVmem),
	VmemTracker_ConvertVmemChunksToMB(curSessionState->sessionVmemRunaway), curSessionState->commandCountRunaway);

	curSessionState = curSessionState->next;
	}
	}
	PG_CATCH();
	{
	LWLockRelease(SessionStateLock);
	PG_RE_THROW();
	}
	PG_END_TRY();

	LWLockRelease(SessionStateLock);
	}