[IMT] ROOT::GetThreadPoolSize does not reflect tbb::global_control settings

[eguiraud]

Describe the bug

Calling tbb::global_control c(tbb::global_control::max_allowed_parallelism, 3); followed by ROOT::GetThreadPoolSize() or TThreadExecutor::GetPoolSize() results in the incorrect number of threads reported (hardware concurrency rather than what was set via tbb::global_control.

Expected behavior

ROOT::GetThreadPoolSize should report the real TBB thread pool size or the real number of threads that ROOT will use.

To Reproduce

This code prints 8 two times on my 8-core workstation, but CPU usage is actually 300%, i.e. TBB's thread pool has size 3 but we ROOT reports 8.

#include <ROOT/TThreadExecutor.hxx>
#include <tbb/global_control.h>
#include <tbb/task_scheduler_init.h>
#include <iostream>
#include <TROOT.h>

int main() {
  tbb::global_control c(tbb::global_control::max_allowed_parallelism, 3);
  ROOT::EnableImplicitMT();
  std::cout << ROOT::GetThreadPoolSize() << std::endl;
  ROOT::TThreadExecutor pool;
  std::cout << pool.GetPoolSize() << std::endl;
  auto busyLoop = [](int x) { int i = 0; for (; i < 1000000000+x; ++i); return i; };
  pool.Foreach(busyLoop, {1,2,3,4,5,6,7,8});
  return 0;
}

[eguiraud]

Maybe we could check tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism) in

root/core/imt/src/RTaskArena.cxx

Lines 69 to 81 in 178dd7f

	RTaskArenaWrapper::RTaskArenaWrapper(unsigned maxConcurrency) : fTBBArena(new tbb::task_arena{})
	{
	const unsigned tbbDefaultNumberThreads = fTBBArena->max_concurrency(); // not initialized, automatic state
	maxConcurrency = maxConcurrency > 0 ? std::min(maxConcurrency, tbbDefaultNumberThreads) : tbbDefaultNumberThreads;
	const unsigned bcCpus = LogicalCPUBandwithControl();
	if (maxConcurrency > bcCpus) {
	Warning("RTaskArenaWrapper", "CPU Bandwith Control Active. Proceeding with %d threads accordingly", bcCpus);
	maxConcurrency = bcCpus;
	}
	fTBBArena->initialize(maxConcurrency);
	fNWorkers = maxConcurrency;
	ROOT::EnableThreadSafety();
	}

[xvallspl]

Makes sense, can do. I would expect tbb::task_arena::max_concurrency() to check that

[eguiraud]

uhm on the other hand tbb::global_control is a RAII kind of thing, so it can come and go. Maybe it's on at the point ROOT's task arena is constructed but not at the point tasks are spawned, and users now get less threads in the arena for "no reason". Not sure.

[xvallspl]

Solved in #6814