updated rebalance conditions,tests

contracts/interfaces/IReserveInterestRateStrategy.sol

@@ -10,9 +10,13 @@ interface IReserveInterestRateStrategy {
   /**
    * @dev returns the base variable borrow rate, in rays
    */
-
   function baseVariableBorrowRate() external view returns (uint256);
 
+  /**
+   * @dev returns the maximum variable borrow rate
+   */
+  function getMaxVariableBorrowRate() external view returns (uint256);
+
   /**
    * @dev calculates the liquidity, stable, and variable rates depending on the current utilization rate
    *      and the base parameters

contracts/lendingpool/DefaultReserveInterestRateStrategy.sol

@@ -89,6 +89,10 @@ contract DefaultReserveInterestRateStrategy is IReserveInterestRateStrategy {
     return _baseVariableBorrowRate;
   }
 
+  function getMaxVariableBorrowRate() external override view returns (uint256) {
+    return _baseVariableBorrowRate.add(_variableRateSlope1).add(_variableRateSlope2);
+  }
+
   /**
    * @dev calculates the interest rates depending on the available liquidity and the total borrowed.
    * @param reserve the address of the reserve

contracts/lendingpool/LendingPool.sol

@@ -12,6 +12,7 @@ import {IAToken} from '../tokenization/interfaces/IAToken.sol';
 import {Helpers} from '../libraries/helpers/Helpers.sol';
 import {Errors} from '../libraries/helpers/Errors.sol';
 import {WadRayMath} from '../libraries/math/WadRayMath.sol';
+import {PercentageMath} from '../libraries/math/PercentageMath.sol';
 import {ReserveLogic} from '../libraries/logic/ReserveLogic.sol';
 import {GenericLogic} from '../libraries/logic/GenericLogic.sol';
 import {ValidationLogic} from '../libraries/logic/ValidationLogic.sol';
@@ -26,6 +27,7 @@ import {IPriceOracleGetter} from '../interfaces/IPriceOracleGetter.sol';
 import {SafeERC20} from '@openzeppelin/contracts/token/ERC20/SafeERC20.sol';
 import {ILendingPool} from '../interfaces/ILendingPool.sol';
 import {LendingPoolStorage} from './LendingPoolStorage.sol';
+import {IReserveInterestRateStrategy} from '../interfaces/IReserveInterestRateStrategy.sol';
 
 /**
  * @title LendingPool contract
@@ -36,10 +38,12 @@ import {LendingPoolStorage} from './LendingPoolStorage.sol';
 contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage {
   using SafeMath for uint256;
   using WadRayMath for uint256;
+  using PercentageMath for uint256;
   using SafeERC20 for IERC20;
 
   //main configuration parameters
-  uint256 public constant REBALANCE_DOWN_RATE_DELTA = (1e27) / 5;
+  uint256 public constant REBALANCE_UP_LIQUIDITY_RATE_THRESHOLD = 4000;
+  uint256 public constant REBALANCE_UP_USAGE_RATIO_THRESHOLD = 0.95 * 1e27; //usage ratio of 95%
   uint256 public constant MAX_STABLE_RATE_BORROW_SIZE_PERCENT = 25;
   uint256 public constant FLASHLOAN_PREMIUM_TOTAL = 9;
 
@@ -354,26 +358,39 @@ contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage
     whenNotPaused();
     ReserveLogic.ReserveData storage reserve = _reserves[asset];
 
-    IStableDebtToken stableDebtToken = IStableDebtToken(reserve.stableDebtTokenAddress);
+    IERC20 stableDebtToken = IERC20(reserve.stableDebtTokenAddress);
+    IERC20 variableDebtToken = IERC20(reserve.variableDebtTokenAddress);
 
-    uint256 stableBorrowBalance = IERC20(address(stableDebtToken)).balanceOf(user);
+    uint256 stableBorrowBalance = IERC20(stableDebtToken).balanceOf(user);
 
     // user must be borrowing on asset at a stable rate
     require(stableBorrowBalance > 0, Errors.NOT_ENOUGH_STABLE_BORROW_BALANCE);
 
-    uint256 rebalanceDownRateThreshold = WadRayMath.ray().add(REBALANCE_DOWN_RATE_DELTA).rayMul(
+    //if the utilization rate is below 95%, no rebalances are needed
-      reserve.currentStableBorrowRate
+    uint256 totalBorrows = stableDebtToken.totalSupply().add(variableDebtToken.totalSupply());
-    );
+    uint256 availableLiquidity = IERC20(reserve.aTokenAddress).totalSupply();
-
+    uint256 utilizationRate = totalBorrows == 0
-    //1. user stable borrow rate is below the current liquidity rate. The loan needs to be rebalanced,
+      ? 0
-    //as this situation can be abused (user putting back the borrowed liquidity in the same reserve to earn on it)
+      : totalBorrows.rayDiv(availableLiquidity.add(totalBorrows));
-    //2. user stable rate is above the market avg borrow rate of a certain delta, and utilization rate is low.
-    //In this case, the user is paying an interest that is too high, and needs to be rescaled down.
-
-    uint256 userStableRate = stableDebtToken.getUserStableRate(user);
 
     require(
-      userStableRate < reserve.currentLiquidityRate || userStableRate > rebalanceDownRateThreshold,
+      utilizationRate >= REBALANCE_UP_USAGE_RATIO_THRESHOLD,
+      Errors.INTEREST_RATE_REBALANCE_CONDITIONS_NOT_MET
+    );
+
+    //if the liquidity rate is below REBALANCE_UP_THRESHOLD of the max variable APR at 95% usage,
+    //then we allow rebalancing of the stable rate positions.
+
+    uint256 currentLiquidityRate = reserve.currentLiquidityRate;
+    uint256 maxVariableBorrowRate = IReserveInterestRateStrategy(
+      reserve
+        .interestRateStrategyAddress
+    )
+      .getMaxVariableBorrowRate();
+
+    require(
+      currentLiquidityRate <=
+        maxVariableBorrowRate.percentMul(REBALANCE_UP_LIQUIDITY_RATE_THRESHOLD),
       Errors.INTEREST_RATE_REBALANCE_CONDITIONS_NOT_MET
     );
 
@@ -381,8 +398,8 @@ contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage
 
     reserve.updateCumulativeIndexesAndTimestamp();
 
-    stableDebtToken.burn(user, stableBorrowBalance);
+    IStableDebtToken(address(stableDebtToken)).burn(user, stableBorrowBalance);
-    stableDebtToken.mint(user, stableBorrowBalance, reserve.currentStableBorrowRate);
+    IStableDebtToken(address(stableDebtToken)).mint(user, stableBorrowBalance, reserve.currentStableBorrowRate);
 
     reserve.updateInterestRates(asset, reserve.aTokenAddress, 0, 0);
 
@@ -507,7 +524,7 @@ contract LendingPool is VersionedInitializable, ILendingPool, LendingPoolStorage
     _flashLiquidationLocked = false;
   }
 
-    struct FlashLoanLocalVars {
+  struct FlashLoanLocalVars {
     uint256 premium;
     uint256 amountPlusPremium;
     IFlashLoanReceiver receiver;

test/helpers/scenarios/rebalance-stable-rate.json

@@ -19,7 +19,7 @@
       ]
     },
     {
-      "description": "User 0 deposits 1000 DAI, user 1 deposits 1 ETH, borrows 100 DAI at a variable rate, user 0 rebalances user 1 (revert expected)",
+      "description": "User 0 deposits 1000 DAI, user 1 deposits 5 ETH, borrows 600 DAI at a variable rate, user 0 rebalances user 1 (revert expected)",
       "actions": [
         {
           "name": "mint",
@@ -51,7 +51,7 @@
           "name": "mint",
           "args": {
             "reserve": "WETH",
-            "amount": "1",
+            "amount": "5",
             "user": "1"
           },
           "expected": "success"
@@ -69,7 +69,7 @@
           "args": {
             "reserve": "WETH",
 
-            "amount": "1",
+            "amount": "5",
             "user": "1"
           },
           "expected": "success"
@@ -78,7 +78,7 @@
           "name": "borrow",
           "args": {
             "reserve": "DAI",
-            "amount": "100",
+            "amount": "600",
             "borrowRateMode": "variable",
             "user": "1",
             "timeTravel": "365"
@@ -122,7 +122,7 @@
       ]
     },
     {
-      "description": "User 2 deposits ETH and borrows the remaining DAI, causing the stable rates to rise (liquidity rate < user 1 borrow rate). User 0 tries to rebalance user 1 (revert expected)",
+      "description": "User 2 deposits ETH and borrows the remaining DAI, causing the stable rates to rise (usage ratio = 94%). User 0 tries to rebalance user 1 (revert expected)",
       "actions": [
         {
           "name": "mint",
@@ -155,7 +155,7 @@
           "name": "borrow",
           "args": {
             "reserve": "DAI",
-            "amount": "100",
+            "amount": "340",
             "borrowRateMode": "variable",
             "user": "2"
           },
@@ -174,40 +174,13 @@
       ]
     },
     {
-      "description": "User 2 borrows more DAI, causing the liquidity rate to rise above user 1 stable borrow rate User 0 rebalances user 1",
+      "description": "User 2 borrows the remaining DAI (usage ratio = 100%). User 0 rebalances user 1",
       "actions": [
-        {
-          "name": "mint",
-          "args": {
-            "reserve": "WETH",
-            "amount": "3",
-            "user": "2"
-          },
-          "expected": "success"
-        },
-        {
-          "name": "approve",
-          "args": {
-            "reserve": "WETH",
-            "user": "2"
-          },
-          "expected": "success"
-        },
-        {
-          "name": "deposit",
-          "args": {
-            "reserve": "WETH",
-
-            "amount": "3",
-            "user": "2"
-          },
-          "expected": "success"
-        },
         {
           "name": "borrow",
           "args": {
             "reserve": "DAI",
-            "amount": "700",
+            "amount": "60",
             "borrowRateMode": "variable",
             "user": "2"
           },