[imt] Introduce env var ROOT_MAX_THREADS:

This overrides the number of threads that ROOT will be using in the thread pool.
This can be set to be higher or lower than the number of physical cores; it can
be specified as decimal, octal, or hex.

Fixes issue #9805.

Mention the env var in `ROOT::EnableImplicitMT()`'s doc.

core/base/src/TROOT.cxx

@@ -521,6 +521,9 @@ namespace Internal {
    /// a hint for ROOT: it will try to satisfy the request if the execution
    /// scenario allows it. For example, if ROOT is configured to use an external
    /// scheduler, setting a value for 'numthreads' might not have any effect.
+   /// To avoid this heuristic, the maximum number of threads can be set by the
+   /// environment variable `ROOT_MAX_THREADS`: `export ROOT_MAX_THREADS=2` will
+   /// set the maximum number of active threads to 2.
    ///
    /// \note Use `DisableImplicitMT()` to disable multi-threading (some locks will remain in place as
    /// described in EnableThreadSafety()). `EnableImplicitMT(1)` creates a thread-pool of size 1.

core/imt/src/RTaskArena.cxx

@@ -5,9 +5,11 @@
 #include "ROpaqueTaskArena.hxx"
 #include "TError.h"
 #include "TROOT.h"
+#include "TSystem.h"
 #include "TThread.h"
 #include <fstream>
 #include <mutex>
+#include <string>
 #include <thread>
 #include "tbb/task_arena.h"
 #define TBB_PREVIEW_GLOBAL_CONTROL 1 // required for TBB versions preceding 2019_U4
@@ -43,9 +45,24 @@
 namespace ROOT {
 namespace Internal {
 
-// To honor cgroup quotas if set: see https://github.com/oneapi-src/oneTBB/issues/190
+// Honor environment variable `ROOT_MAX_THREADS` if set.
+// Also honor cgroup quotas if set: see https://github.com/oneapi-src/oneTBB/issues/190
 int LogicalCPUBandwidthControl()
 {
+   if (const char *envMaxThreads = gSystem->Getenv("ROOT_MAX_THREADS")) {
+      char *str_end = nullptr;
+      long maxThreads = std::strtol(envMaxThreads, &str_end, 0 /*auto-detect base*/);
+      if (str_end == envMaxThreads && maxThreads == 0) {
+         Error("ROOT::Internal::LogicalCPUBandwidthControl()",
+               "cannot parse number in environment variable ROOT_MAX_THREADS; ignoring.");
+      } else if (maxThreads < 1) {
+         Error("ROOT::Internal::LogicalCPUBandwidthControl()",
+               "environment variable ROOT_MAX_THREADS must be >= 1, but set to %ld; ignoring.",
+               maxThreads);
+      } else
+         return maxThreads;
+   }
+
 #ifdef R__LINUX
    // Check for CFS bandwith control
    std::ifstream f("/sys/fs/cgroup/cpuacct/cpu.cfs_quota_us"); // quota file