sqlstats: convert RWMutex to Mutex

This commit changes the RWMutex on the sql stats
containers to the regular exclusive Mutex struct.
The motivation behind this change is that RWMutex
scales poorly with CPU count. See benchmarks below.

Epic: none
Part of: #140590

Release note: None

Author: xinhaoz

pkg/sql/sqlstats/ssmemstorage/ss_mem_iterator.go

@@ -32,8 +32,8 @@ func NewStmtStatsIterator(
 ) StmtStatsIterator {
 	var stmtKeys stmtList
 	func() {
-		container.mu.RLock()
-		defer container.mu.RUnlock()
+		container.mu.Lock()
+		defer container.mu.Unlock()
 		for k := range container.mu.stmts {
 			stmtKeys = append(stmtKeys, k)
 		}

pkg/sql/sqlstats/ssmemstorage/ss_mem_storage.go

@@ -69,7 +69,7 @@ type Container struct {
 	uniqueServerCount *SQLStatsAtomicCounters
 
 	mu struct {
-		syncutil.RWMutex
+		syncutil.Mutex
 
 		// acc is the memory account that tracks memory allocations related to stmts
 		// and txns within this Container struct.
@@ -431,8 +431,8 @@ func (s *stmtStats) mergeStatsLocked(statistics *appstatspb.CollectedStatementSt
 
 // getStatsForStmtWithKey returns an instance of stmtStats.
 func (s *Container) getStatsForStmtWithKey(key stmtKey) (stats *stmtStats) {
-	s.mu.RLock()
-	defer s.mu.RUnlock()
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	stats = s.mu.stmts[key]
 	return stats
 }
@@ -480,8 +480,8 @@ func (s *Container) tryCreateStatsForStmtWithKeyLocked(
 }
 
 func (s *Container) getStatsForTxnWithKey(key appstatspb.TransactionFingerprintID) *txnStats {
-	s.mu.RLock()
-	defer s.mu.RUnlock()
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	return s.mu.txns[key]
 }
 

pkg/sql/sqlstats/ssmemstorage/ss_mem_writer.go

@@ -163,8 +163,8 @@ func (s *Container) StatementSampled(fingerprint string, implicitTxn bool, datab
 		implicitTxn:     implicitTxn,
 		database:        database,
 	}
-	s.mu.RLock()
-	defer s.mu.RUnlock()
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	_, ok := s.mu.sampledStatementCache[key]
 	return ok
 }