2 files changed, 43 insertions, 7 deletions
diff --git a/src/corelib/thread/qthread.cpp b/src/corelib/thread/qthread.cpp
index a936d189f4c..4cfcab2258d 100644
--- a/src/corelib/thread/qthread.cpp
+++ b/src/corelib/thread/qthread.cpp
@@ -314,9 +314,20 @@ QThreadPrivate::~QThreadPrivate()
 /*!
     \fn int QThread::idealThreadCount()
 
-    Returns the ideal number of threads that can be run on the system. This is done querying
-    the number of processor cores, both real and logical, in the system. This function returns 1
-    if the number of processor cores could not be detected.
+    Returns the ideal number of threads that this process can run in parallel.
+    This is done by querying the number of logical processors available to this
+    process (if supported by this OS) or the total number of logical processors
+    in the system. This function returns 1 if neither value could be
+    determined.
+
+    \note On operating systems that support setting a thread's affinity to a
+    subset of all logical processors, the value returned by this function may
+    change between threads and over time.
+
+    \note On operating systems that support CPU hotplugging and hot-unplugging,
+    the value returned by this function may also change over time (and note
+    that CPUs can be turned on and off by software, without a physical,
+    hardware change).
 */
 
 /*!
diff --git a/src/corelib/thread/qthread_unix.cpp b/src/corelib/thread/qthread_unix.cpp
index 8fa4e939f83..79abfe7e500 100644
--- a/src/corelib/thread/qthread_unix.cpp
+++ b/src/corelib/thread/qthread_unix.cpp
@@ -70,8 +70,10 @@
 #include <sched.h>
 #include <errno.h>
 
-#ifdef Q_OS_BSD4
-#include <sys/sysctl.h>
+#if defined(Q_OS_FREEBSD)
+#  include <sys/cpuset.h>
+#elif defined(Q_OS_BSD4)
+#  include <sys/sysctl.h>
 #endif
 #ifdef Q_OS_VXWORKS
 #  if (_WRS_VXWORKS_MAJOR > 6) || ((_WRS_VXWORKS_MAJOR == 6) && (_WRS_VXWORKS_MINOR >= 6))
@@ -455,8 +457,31 @@ int QThread::idealThreadCount() noexcept
     } else {
         cores = (int)psd.psd_proc_cnt;
     }
+#elif (defined(Q_OS_LINUX) && !defined(Q_OS_ANDROID)) || defined(Q_OS_FREEBSD)
+#  ifdef Q_OS_FREEBSD
+#    define CPU_COUNT_S(setsize, cpusetp)   ((int)BIT_COUNT(setsize, cpusetp))
+    // match the Linux API for simplicity
+    using cpu_set_t = cpuset_t;
+    auto sched_getaffinity = [](pid_t, size_t cpusetsize, cpu_set_t *mask) {
+        return cpuset_getaffinity(CPU_LEVEL_WHICH, CPU_WHICH_PID, -1, cpusetsize, mask);
+    };
+#  endif
+
+    // get the number of threads we're assigned, not the total in the system
+    QVarLengthArray<cpu_set_t, 1> cpuset(1);
+    int size = 1;
+    if (Q_UNLIKELY(sched_getaffinity(0, sizeof(cpu_set_t), cpuset.data()) < 0)) {
+        for (size = 2; size <= 4; size *= 2) {
+            cpuset.resize(size);
+            if (sched_getaffinity(0, sizeof(cpu_set_t) * size, cpuset.data()) == 0)
+                break;
+        }
+        if (size > 4)
+            return 1;
+    }
+    cores = CPU_COUNT_S(sizeof(cpu_set_t) * size, cpuset.data());
 #elif defined(Q_OS_BSD4)
-    // FreeBSD, OpenBSD, NetBSD, BSD/OS, OS X, iOS
+    // OpenBSD, NetBSD, BSD/OS, Darwin (macOS, iOS, etc.)
     size_t len = sizeof(cores);
     int mib[2];
     mib[0] = CTL_HW;
@@ -494,7 +519,7 @@ int QThread::idealThreadCount() noexcept
 #elif defined(Q_OS_WASM)
     cores = QThreadPrivate::idealThreadCount;
 #else
-    // the rest: Linux, Solaris, AIX, Tru64
+    // the rest: Solaris, AIX, Tru64
     cores = (int)sysconf(_SC_NPROCESSORS_ONLN);
     if (cores == -1)
         return 1;