From 0c31264ae68785095d7678e5ee60f0378dae5b93 Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Thu, 27 Oct 2022 12:40:12 +1300
Subject: [PATCH v3 4/6] Use SetLatches() for SERIALIZABLE DEFERRABLE wakeups.

Don't issue SetLatch()'s system call while holding a highly contended
LWLock.  Collect them in a LatchGroup to be set after the lock is
released.  Once woken, other backends will immediately try to acquire
that lock anyway so it's better to wake after releasing.

Take this opportunity to retire the confusingly named ProcSendSignal()
and replace it with something that gives the latch pointer we need.
There was only one other caller, in bufmgr.c, which is easily changed.

Discussion: https://postgr.es/m/CA%2BhUKGKmO7ze0Z6WXKdrLxmvYa%3DzVGGXOO30MMktufofVwEm1A%40mail.gmail.com
---
 src/backend/storage/buffer/bufmgr.c  |  2 +-
 src/backend/storage/ipc/procsignal.c |  2 --
 src/backend/storage/lmgr/predicate.c |  5 ++++-
 src/backend/storage/lmgr/proc.c      | 12 ------------
 src/include/storage/proc.h           |  4 +++-
 5 files changed, 8 insertions(+), 17 deletions(-)

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 0a05577b68..f297958f1b 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -1966,7 +1966,7 @@ UnpinBuffer(BufferDesc *buf)
 
 				buf_state &= ~BM_PIN_COUNT_WAITER;
 				UnlockBufHdr(buf, buf_state);
-				ProcSendSignal(wait_backend_pgprocno);
+				SetLatch(GetProcLatchByNumber(wait_backend_pgprocno));
 			}
 			else
 				UnlockBufHdr(buf, buf_state);
diff --git a/src/backend/storage/ipc/procsignal.c b/src/backend/storage/ipc/procsignal.c
index 395b2cf690..13ec2317fc 100644
--- a/src/backend/storage/ipc/procsignal.c
+++ b/src/backend/storage/ipc/procsignal.c
@@ -255,8 +255,6 @@ CleanupProcSignalState(int status, Datum arg)
  *
  * On success (a signal was sent), zero is returned.
  * On error, -1 is returned, and errno is set (typically to ESRCH or EPERM).
- *
- * Not to be confused with ProcSendSignal
  */
 int
 SendProcSignal(pid_t pid, ProcSignalReason reason, BackendId backendId)
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 0f7f7ba5f1..86b046999b 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -3235,6 +3235,7 @@ ReleasePredicateLocks(bool isCommit, bool isReadOnlySafe)
 	bool		needToClear;
 	SERIALIZABLEXACT *roXact;
 	dlist_mutable_iter iter;
+	LatchGroup	wakeups = {0};
 
 	/*
 	 * We can't trust XactReadOnly here, because a transaction which started
@@ -3538,7 +3539,7 @@ ReleasePredicateLocks(bool isCommit, bool isReadOnlySafe)
 			 */
 			if (SxactIsDeferrableWaiting(roXact) &&
 				(SxactIsROUnsafe(roXact) || SxactIsROSafe(roXact)))
-				ProcSendSignal(roXact->pgprocno);
+				AddLatch(&wakeups, GetProcLatchByNumber(roXact->pgprocno));
 		}
 	}
 
@@ -3562,6 +3563,8 @@ ReleasePredicateLocks(bool isCommit, bool isReadOnlySafe)
 
 	LWLockRelease(SerializableXactHashLock);
 
+	SetLatches(&wakeups);
+
 	LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
 
 	/* Add this to the list of transactions to check for later cleanup. */
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 22b4278610..09eda65fd1 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -1802,18 +1802,6 @@ ProcWaitForSignal(uint32 wait_event_info)
 	CHECK_FOR_INTERRUPTS();
 }
 
-/*
- * ProcSendSignal - set the latch of a backend identified by pgprocno
- */
-void
-ProcSendSignal(int pgprocno)
-{
-	if (pgprocno < 0 || pgprocno >= ProcGlobal->allProcCount)
-		elog(ERROR, "pgprocno out of range");
-
-	SetLatch(&ProcGlobal->allProcs[pgprocno].procLatch);
-}
-
 /*
  * BecomeLockGroupLeader - designate process as lock group leader
  *
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 4258cd92c9..ca39ae9486 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -413,6 +413,9 @@ extern PGDLLIMPORT PGPROC *PreparedXactProcs;
 /* Accessor for PGPROC given a pgprocno. */
 #define GetPGProcByNumber(n) (&ProcGlobal->allProcs[(n)])
 
+/* Accessor for procLatch given a pgprocno. */
+#define GetProcLatchByNumber(n) (&(GetPGProcByNumber(n))->procLatch)
+
 /*
  * We set aside some extra PGPROC structures for auxiliary processes,
  * ie things that aren't full-fledged backends but need shmem access.
@@ -456,7 +459,6 @@ extern bool IsWaitingForLock(void);
 extern void LockErrorCleanup(void);
 
 extern void ProcWaitForSignal(uint32 wait_event_info);
-extern void ProcSendSignal(int pgprocno);
 
 extern PGPROC *AuxiliaryPidGetProc(int pid);
 
-- 
2.39.2