diff --git a/src/common/hist_util.cc b/src/common/hist_util.cc
index 6431487bb4b6..71cc76611581 100644
--- a/src/common/hist_util.cc
+++ b/src/common/hist_util.cc
@@ -259,10 +259,11 @@ GHistBuilder<double>::BuildHist<false>(const std::vector<GradientPair> &gpair,
                                        const GHistIndexMatrix &gmat,
                                        GHistRow<double> hist) const;
 
-template<typename GradientSumT>
+template <typename GradientSumT>
 void GHistBuilder<GradientSumT>::SubtractionTrick(GHistRowT self,
                                                   GHistRowT sibling,
-                                                  GHistRowT parent) {
+                                                  GHistRowT parent,
+                                                  int32_t n_threads) {
   const size_t size = self.size();
   CHECK_EQ(sibling.size(), size);
   CHECK_EQ(parent.size(), size);
@@ -270,7 +271,7 @@ void GHistBuilder<GradientSumT>::SubtractionTrick(GHistRowT self,
   const size_t block_size = 1024;  // aproximatly 1024 values per block
   size_t n_blocks = size/block_size + !!(size%block_size);
 
-  ParallelFor(omp_ulong(n_blocks), [&](omp_ulong iblock) {
+  ParallelFor(omp_ulong(n_blocks), n_threads, [&](omp_ulong iblock) {
     const size_t ibegin = iblock * block_size;
     const size_t iend =
         (((iblock + 1) * block_size > size) ? size : ibegin + block_size);
@@ -280,11 +281,13 @@ void GHistBuilder<GradientSumT>::SubtractionTrick(GHistRowT self,
 template
 void GHistBuilder<float>::SubtractionTrick(GHistRow<float> self,
                                            GHistRow<float> sibling,
-                                           GHistRow<float> parent);
+                                           GHistRow<float> parent,
+                                           int32_t n_threads);
 template
 void GHistBuilder<double>::SubtractionTrick(GHistRow<double> self,
                                             GHistRow<double> sibling,
-                                            GHistRow<double> parent);
+                                            GHistRow<double> parent,
+                                            int32_t n_threads);
 
 }  // namespace common
 }  // namespace xgboost
diff --git a/src/common/hist_util.h b/src/common/hist_util.h
index 1fa3a2240258..3cc130e8135d 100644
--- a/src/common/hist_util.h
+++ b/src/common/hist_util.h
@@ -567,26 +567,22 @@ class GHistBuilder {
   using GHistRowT = GHistRow<GradientSumT>;
 
   GHistBuilder() = default;
-  GHistBuilder(size_t nthread, uint32_t nbins) : nthread_{nthread}, nbins_{nbins} {}
+  explicit GHistBuilder(uint32_t nbins): nbins_{nbins} {}
 
   // construct a histogram via histogram aggregation
   template <bool any_missing>
-  void BuildHist(const std::vector<GradientPair>& gpair,
+  void BuildHist(const std::vector<GradientPair> &gpair,
                  const RowSetCollection::Elem row_indices,
-                 const GHistIndexMatrix& gmat,
-                 GHistRowT hist) const;
+                 const GHistIndexMatrix &gmat, GHistRowT hist) const;
   // construct a histogram via subtraction trick
-  void SubtractionTrick(GHistRowT self,
-                        GHistRowT sibling,
-                        GHistRowT parent);
+  void SubtractionTrick(GHistRowT self, GHistRowT sibling, GHistRowT parent,
+                        int32_t n_threads);
 
   uint32_t GetNumBins() const {
       return nbins_;
   }
 
  private:
-  /*! \brief number of threads for parallel computation */
-  size_t nthread_ { 0 };
   /*! \brief number of all bins over all features */
   uint32_t nbins_ { 0 };
 };
diff --git a/src/common/partition_builder.h b/src/common/partition_builder.h
index 5ffe34988968..0a59b6522428 100644
--- a/src/common/partition_builder.h
+++ b/src/common/partition_builder.h
@@ -1,4 +1,3 @@
-
 /*!
  * Copyright 2021 by Contributors
  * \file row_set.h
@@ -236,7 +235,7 @@ class PartitionBuilder {
     return blocks_offsets_[nid] + begin / BlockSize;
   }
 
- protected:
+ private:
   struct BlockInfo{
     size_t n_left;
     size_t n_right;
diff --git a/src/tree/hist/histogram.h b/src/tree/hist/histogram.h
index 2352310d9b9e..0ff16fdc60fe 100644
--- a/src/tree/hist/histogram.h
+++ b/src/tree/hist/histogram.h
@@ -51,7 +51,7 @@ template <typename GradientSumT, typename ExpandEntry> class HistogramBuilder {
     hist_.Init(total_bins);
     hist_local_worker_.Init(total_bins);
     buffer_.Init(total_bins);
-    builder_ = common::GHistBuilder<GradientSumT>(n_threads, total_bins);
+    builder_ = common::GHistBuilder<GradientSumT>(total_bins);
     is_distributed_ = is_distributed;
   }
 
diff --git a/tests/cpp/tree/hist/test_evaluate_splits.cc b/tests/cpp/tree/hist/test_evaluate_splits.cc
index cb0171269305..fe7b59ccee46 100644
--- a/tests/cpp/tree/hist/test_evaluate_splits.cc
+++ b/tests/cpp/tree/hist/test_evaluate_splits.cc
@@ -39,7 +39,7 @@ template <typename GradientSumT> void TestEvaluateSplits() {
   std::iota(row_indices.begin(), row_indices.end(), 0);
   row_set_collection.Init();
 
-  auto hist_builder = GHistBuilder<GradientSumT>(n_threads, gmat.cut.Ptrs().back());
+  auto hist_builder = GHistBuilder<GradientSumT>(gmat.cut.Ptrs().back());
   hist.Init(gmat.cut.Ptrs().back());
   hist.AddHistRow(0);
   hist.AllocateAllData();