reduce unnecessary tikvServerBusy backoff when able to try next replica

internal/locate/region_request.go

@@ -1265,21 +1265,15 @@ func (s *replicaSelector) onServerIsBusy(
 				// Clear attempt history of the leader, so the leader can be accessed again.
 				s.replicas[state.leaderIdx].attempts = 0
 				s.state = &tryIdleReplica{leaderIdx: state.leaderIdx}
-				return true, nil
-			case *tryIdleReplica:
-				if s.targetIdx != state.leaderIdx {
-					return true, nil
-				}
-				// backoff if still receiving ServerIsBusy after accessing leader again
 			}
 		}
 	} else if ctx != nil && ctx.Store != nil {
 		// Mark the server is busy (the next incoming READs could be redirect
 		// to expected followers. )
 		ctx.Store.markAlreadySlow()
-		if s.canFallback2Follower() {
-			return true, nil
-		}
+	}
+	if s.canSkipServerIsBusyBackoff() {
+		return true, nil
 	}
 	err = bo.Backoff(retry.BoTiKVServerBusy, errors.Errorf("server is busy, ctx: %v", ctx))
 	if err != nil {
@@ -1288,21 +1282,24 @@ func (s *replicaSelector) onServerIsBusy(
 	return true, nil
 }
 
-// For some reasons, the leader is unreachable by now, try followers instead.
-// the state is changed in accessFollower.next when leader is unavailable.
-func (s *replicaSelector) canFallback2Follower() bool {
+// canSkipServerIsBusyBackoff returns true if the request can be sent to next replica and can skip ServerIsBusy backoff.
+func (s *replicaSelector) canSkipServerIsBusyBackoff() bool {
 	if s == nil || s.state == nil {
 		return false
 	}
-	state, ok := s.state.(*accessFollower)
-	if !ok {
+	accessLeader, ok := s.state.(*accessKnownLeader)
+	if ok && accessLeader.isCandidate(s.replicas[accessLeader.leaderIdx]) {
+		// If leader is still candidate, the request will be sent to leader again,
+		// so don't skip since the leader is still busy.
 		return false
 	}
-	if !state.isStaleRead {
-		return false
+	for _, replica := range s.replicas {
+		if replica.attempts == 0 && replica.store.getLivenessState() == reachable &&
+			!replica.isEpochStale() && !replica.store.isSlow() {
+			return true
+		}
 	}
-	// can fallback to follower only when the leader is exhausted.
-	return state.lastIdx == state.leaderIdx && state.IsLeaderExhausted(s.replicas[state.leaderIdx])
+	return false
 }
 
 func (s *replicaSelector) onDataIsNotReady() {

internal/locate/region_request3_test.go

@@ -373,6 +373,12 @@ func refreshLivenessStates(regionStore *regionStore) {
 	}
 }
 
+func refreshStoreScores(regionStore *regionStore) {
+	for _, store := range regionStore.stores {
+		store.slowScore.resetSlowScore()
+	}
+}
+
 func AssertRPCCtxEqual(s *testRegionRequestToThreeStoresSuite, rpcCtx *RPCContext, target *replica, proxy *replica) {
 	s.Equal(rpcCtx.Store, target.store)
 	s.Equal(rpcCtx.Peer, target.peer)

internal/locate/replica_selector_test.go

@@ -114,8 +114,8 @@ func TestReplicaReadAccessPathByCase(t *testing.T) {
 			},
 			respErr:         "",
 			respRegionError: nil,
-			backoffCnt:      1,
-			backoffDetail:   []string{"tikvServerBusy+1"},
+			backoffCnt:      0,
+			backoffDetail:   []string{},
 			regionIsValid:   true,
 		},
 	}
@@ -319,8 +319,8 @@ func TestReplicaReadAccessPathByCase(t *testing.T) {
 				"{addr: store2, replica-read: true, stale-read: false}"},
 			respErr:         "",
 			respRegionError: nil,
-			backoffCnt:      1,
-			backoffDetail:   []string{"tikvServerBusy+1"},
+			backoffCnt:      0,
+			backoffDetail:   []string{},
 			regionIsValid:   true,
 		},
 	}
@@ -341,8 +341,8 @@ func TestReplicaReadAccessPathByCase(t *testing.T) {
 			},
 			respErr:         "",
 			respRegionError: fakeEpochNotMatch,
-			backoffCnt:      2,
-			backoffDetail:   []string{"tikvServerBusy+2"},
+			backoffCnt:      1,
+			backoffDetail:   []string{"tikvServerBusy+1"},
 			regionIsValid:   false,
 		},
 	}
@@ -385,14 +385,99 @@ func TestReplicaReadAccessPathByCase(t *testing.T) {
 				"{addr: store3, replica-read: true, stale-read: false}"},
 			respErr:         "",
 			respRegionError: nil,
-			backoffCnt:      1,
-			backoffDetail:   []string{"tikvServerBusy+1"},
+			backoffCnt:      0,
+			backoffDetail:   []string{},
+			regionIsValid:   true,
+		},
+	}
+	s.True(s.runCaseAndCompare(ca))
+
+	ca = replicaSelectorAccessPathCase{
+		reqType:   tikvrpc.CmdGet,
+		readType:  kv.ReplicaReadMixed,
+		staleRead: true,
+		timeout:   time.Microsecond * 100,
+		accessErr: []RegionErrorType{ServerIsBusyErr, ServerIsBusyWithEstimatedWaitMsErr},
+		expect: &accessPathResult{
+			accessPath: []string{
+				"{addr: store1, replica-read: false, stale-read: true}",
+				"{addr: store2, replica-read: true, stale-read: false}",
+				"{addr: store3, replica-read: true, stale-read: false}",
+			},
+			respErr:         "",
+			respRegionError: nil,
+			backoffCnt:      0,
+			backoffDetail:   []string{},
 			regionIsValid:   true,
 		},
 	}
 	s.True(s.runCaseAndCompare(ca))
 }
 
+func TestCanSkipServerIsBusyBackoff(t *testing.T) {
+	s := new(testReplicaSelectorSuite)
+	s.SetupTest(t)
+	defer s.TearDownTest()
+
+	loc, err := s.cache.LocateKey(s.bo, []byte("key"))
+	s.Nil(err)
+	req := tikvrpc.NewRequest(tikvrpc.CmdGet, &kvrpcpb.GetRequest{Key: []byte("key")})
+	req.EnableStaleWithMixedReplicaRead()
+	selector, err := newReplicaSelector(s.cache, loc.Region, req)
+	s.Nil(err)
+	for i := 0; i < 3; i++ {
+		_, err = selector.next(s.bo)
+		s.Nil(err)
+		skip := selector.canSkipServerIsBusyBackoff()
+		if i < 2 {
+			s.True(skip)
+		} else {
+			s.False(skip)
+		}
+	}
+
+	selector, err = newReplicaSelector(s.cache, loc.Region, req)
+	s.Nil(err)
+	_, err = selector.next(s.bo)
+	s.Nil(err)
+	// mock all replica's store is unreachable.
+	for _, replica := range selector.replicas {
+		atomic.StoreUint32(&replica.store.livenessState, uint32(unreachable))
+	}
+	skip := selector.canSkipServerIsBusyBackoff()
+	s.False(skip) // can't skip since no replica is available.
+	refreshLivenessStates(selector.regionStore)
+	skip = selector.canSkipServerIsBusyBackoff()
+	s.True(skip)
+	// mock all replica's store is slow.
+	for _, replica := range selector.replicas {
+		replica.store.markAlreadySlow()
+	}
+	skip = selector.canSkipServerIsBusyBackoff()
+	s.False(skip)
+	refreshStoreScores(selector.regionStore)
+	skip = selector.canSkipServerIsBusyBackoff()
+	s.True(skip)
+
+	// Test for leader read.
+	req = tikvrpc.NewRequest(tikvrpc.CmdGet, &kvrpcpb.GetRequest{Key: []byte("key")})
+	req.ReplicaReadType = kv.ReplicaReadLeader
+	selector, err = newReplicaSelector(s.cache, loc.Region, req)
+	s.Nil(err)
+	for i := 0; i < 12; i++ {
+		_, err = selector.next(s.bo)
+		s.Nil(err)
+		skip := selector.canSkipServerIsBusyBackoff()
+		if i <= 8 {
+			s.False(skip) // can't skip since leader is available.
+		} else if i <= 10 {
+			s.True(skip)
+		} else {
+			s.False(skip)
+		}
+	}
+}
+
 func (s *testReplicaSelectorSuite) changeRegionLeader(storeId uint64) {
 	loc, err := s.cache.LocateKey(s.bo, []byte("key"))
 	s.Nil(err)