diff --git a/mace/kernels/activation.h b/mace/kernels/activation.h
index 0a856fc945b3ce2eebc7eaa8e9d0f6fc4985629a..83acf4fb70311c710bcde1d7b08c1e6c4630879f 100644
--- a/mace/kernels/activation.h
+++ b/mace/kernels/activation.h
@@ -135,6 +135,7 @@ class ActivationFunctor<DeviceType::OPENCL, T> {
   T relux_max_limit_;
   T prelu_alpha_;
   cl::Kernel kernel_;
+  std::string tuning_key_prefix_;
 };
 
 }  // namespace kernels
diff --git a/mace/kernels/opencl/activation_opencl.cc b/mace/kernels/opencl/activation_opencl.cc
index 50ad306368f503860cf4d6688bd10b74fd678d9b..934ce8da666b8e9910815b9f56de250a3511fd68 100644
--- a/mace/kernels/opencl/activation_opencl.cc
+++ b/mace/kernels/opencl/activation_opencl.cc
@@ -22,7 +22,6 @@ void ActivationFunctor<DeviceType::OPENCL, T>::operator()(const Tensor *input,
   const index_t channels = input->dim(3);
 
   const index_t channel_blocks = RoundUpDiv4(channels);
-  std::string tuning_key_prefix;
 
   if (kernel_.get() == nullptr) {
     auto runtime = OpenCLRuntime::Global();
@@ -35,23 +34,23 @@ void ActivationFunctor<DeviceType::OPENCL, T>::operator()(const Tensor *input,
     built_options.emplace("-DCMD_DATA_TYPE=" + DtToUpstreamCLCMDDt(dt));
     switch (activation_) {
       case RELU:
-        tuning_key_prefix = "relu_opencl_kernel_";
+        tuning_key_prefix_ = "relu_opencl_kernel_";
         built_options.emplace("-DUSE_RELU");
         break;
       case RELUX:
-        tuning_key_prefix = "relux_opencl_kernel_";
+        tuning_key_prefix_ = "relux_opencl_kernel_";
         built_options.emplace("-DUSE_RELUX");
         break;
       case PRELU:
-        tuning_key_prefix = "prelu_opencl_kernel_";
+        tuning_key_prefix_ = "prelu_opencl_kernel_";
         built_options.emplace("-DUSE_PRELU");
         break;
       case TANH:
-        tuning_key_prefix = "tanh_opencl_kernel_";
+        tuning_key_prefix_ = "tanh_opencl_kernel_";
         built_options.emplace("-DUSE_TANH");
         break;
       case SIGMOID:
-        tuning_key_prefix = "sigmoid_opencl_kernel_";
+        tuning_key_prefix_ = "sigmoid_opencl_kernel_";
         built_options.emplace("-DUSE_SIGMOID");
         break;
       default:
@@ -60,12 +59,10 @@ void ActivationFunctor<DeviceType::OPENCL, T>::operator()(const Tensor *input,
     kernel_ =
         runtime->BuildKernel("activation", kernel_name, built_options);
     int idx = 0;
-    kernel_.setArg(
-        idx++, *(static_cast<const cl::Image2D *>(input->buffer())));
+    kernel_.setArg(idx++, *(static_cast<const cl::Image2D *>(input->buffer())));
     kernel_.setArg(idx++, static_cast<float>(relux_max_limit_));
     kernel_.setArg(idx++, static_cast<float>(prelu_alpha_));
-    kernel_.setArg(idx++,
-                             *(static_cast<cl::Image2D *>(output->buffer())));
+    kernel_.setArg(idx++, *(static_cast<cl::Image2D *>(output->buffer())));
   }
 
   const uint32_t gws[3] = {static_cast<uint32_t>(channel_blocks),
@@ -73,7 +70,7 @@ void ActivationFunctor<DeviceType::OPENCL, T>::operator()(const Tensor *input,
                            static_cast<uint32_t>(height * batch)};
   const std::vector<uint32_t> lws = {8, 16, 8, 1};
   std::string tuning_key =
-      Concat(tuning_key_prefix, output->dim(0), output->dim(1),
+      Concat(tuning_key_prefix_, output->dim(0), output->dim(1),
              output->dim(2), output->dim(3));
   TuningOrRun3DKernel(kernel_, tuning_key, gws, lws, future);
 }