From 14aa8bcf2091ffa1aef6911f52647e0158d4b628 Mon Sep 17 00:00:00 2001
From: Xavier Valls Pla <xavier.valls.pla@cern.ch>
Date: Mon, 16 Nov 2020 09:51:43 +0100
Subject: [PATCH]     warn on tbb::global_control interferences

    warn when the number of threads set by the user is limited at runtime
    by tbb::global_control.

    Fix for github issue #6363: https://github.com/root-project/root/issues/6363
---
 core/imt/src/RTaskArena.cxx      |  5 +++++
 core/imt/src/TThreadExecutor.cxx | 18 ++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/core/imt/src/RTaskArena.cxx b/core/imt/src/RTaskArena.cxx
index 477dfc932b6ab..883cee96d870f 100644
--- a/core/imt/src/RTaskArena.cxx
+++ b/core/imt/src/RTaskArena.cxx
@@ -6,6 +6,7 @@
 #include <mutex>
 #include <thread>
 #include "tbb/task_arena.h"
+#include "tbb/global_control.h"
 
 //////////////////////////////////////////////////////////////////////////
 ///
@@ -75,6 +76,10 @@ RTaskArenaWrapper::RTaskArenaWrapper(unsigned maxConcurrency) : fTBBArena(new tb
       Warning("RTaskArenaWrapper", "CPU Bandwith Control Active. Proceeding with %d threads accordingly", bcCpus);
       maxConcurrency = bcCpus;
    }
+   if (maxConcurrency > tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism)) {
+      Warning("RTaskArenaWrapper", "tbb::global_control is active, limiting the number of parallel workers"
+                                   "from this task arena available for execution.");
+   }
    fTBBArena->initialize(maxConcurrency);
    fNWorkers = maxConcurrency;
    ROOT::EnableThreadSafety();
diff --git a/core/imt/src/TThreadExecutor.cxx b/core/imt/src/TThreadExecutor.cxx
index 13397af671845..4c20075c03f4a 100644
--- a/core/imt/src/TThreadExecutor.cxx
+++ b/core/imt/src/TThreadExecutor.cxx
@@ -155,6 +155,12 @@ TThreadExecutor::TThreadExecutor(UInt_t nThreads)
 void TThreadExecutor::ParallelFor(unsigned int start, unsigned int end, unsigned step,
                                   const std::function<void(unsigned int i)> &f)
 {
+   if (GetPoolSize() > tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism)) {
+      Warning("TThreadExecutor::ParallelFor",
+              "tbb::global_control is limiting the number of parallel workers."
+              " Proceeding with %zu threads this time",
+              tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism));
+   }
    fTaskArenaW->Access().execute([&] {
       tbb::this_task_arena::isolate([&] {
          tbb::parallel_for(start, end, step, f);
@@ -171,6 +177,12 @@ void TThreadExecutor::ParallelFor(unsigned int start, unsigned int end, unsigned
 double TThreadExecutor::ParallelReduce(const std::vector<double> &objs,
                                        const std::function<double(double a, double b)> &redfunc)
 {
+   if (GetPoolSize() > tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism)) {
+      Warning("TThreadExecutor::ParallelReduce",
+              "tbb::global_control is limiting the number of parallel workers."
+              " Proceeding with %zu threads this time",
+              tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism));
+   }
    return fTaskArenaW->Access().execute([&] { return ROOT::Internal::ParallelReduceHelper<double>(objs, redfunc); });
 }
 
@@ -183,6 +195,12 @@ double TThreadExecutor::ParallelReduce(const std::vector<double> &objs,
 float TThreadExecutor::ParallelReduce(const std::vector<float> &objs,
                                       const std::function<float(float a, float b)> &redfunc)
 {
+   if (GetPoolSize() > tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism)) {
+      Warning("TThreadExecutor::ParallelReduce",
+              "tbb::global_control is limiting the number of parallel workers."
+              " Proceeding with %zu threads this time",
+              tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism));
+   }
    return fTaskArenaW->Access().execute([&] { return ROOT::Internal::ParallelReduceHelper<float>(objs, redfunc); });
 }