--- a/src/os/linux/vm/globals_linux.hpp	Thu Jul 27 16:57:32 2017 +0100
+++ b/src/os/linux/vm/globals_linux.hpp	Thu Apr 27 05:04:03 2017 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2016, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -47,7 +47,10 @@
           "Load DLLs with executable-stack attribute in the VM Thread") \
                                                                         \
   product(bool, UseSHM, false,                                          \
-          "Use SYSV shared memory for large pages")
+          "Use SYSV shared memory for large pages")                     \
+                                                                        \
+  diagnostic(bool, PrintActiveCpus, false,                              \
+          "Print the number of CPUs detected in os::active_processor_count")
 
 //
 // Defines Linux-specific default values. The flags are available on all

--- a/src/os/linux/vm/os_linux.cpp	Thu Jul 27 16:57:32 2017 +0100
+++ b/src/os/linux/vm/os_linux.cpp	Thu Apr 27 05:04:03 2017 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1999, 2015, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1999, 2016, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -124,6 +124,14 @@
 # include <inttypes.h>
 # include <sys/ioctl.h>
 
+#ifndef _GNU_SOURCE
+  #define _GNU_SOURCE
+  #include <sched.h>
+  #undef _GNU_SOURCE
+#else
+  #include <sched.h>
+#endif
+
 // if RUSAGE_THREAD for getrusage() has not been defined, do it here. The code calling
 // getrusage() is prepared to handle the associated failure.
 #ifndef RUSAGE_THREAD
@@ -5085,12 +5093,42 @@
   }
 };
 
+static int os_cpu_count(const cpu_set_t* cpus) {
+  int count = 0;
+  // only look up to the number of configured processors
+  for (int i = 0; i < os::processor_count(); i++) {
+    if (CPU_ISSET(i, cpus)) {
+      count++;
+    }
+  }
+  return count;
+}
+
+// Get the current number of available processors for this process.
+// This value can change at any time during a process's lifetime.
+// sched_getaffinity gives an accurate answer as it accounts for cpusets.
+// If anything goes wrong we fallback to returning the number of online
+// processors - which can be greater than the number available to the process.
 int os::active_processor_count() {
-  // Linux doesn't yet have a (official) notion of processor sets,
-  // so just return the number of online processors.
-  int online_cpus = ::sysconf(_SC_NPROCESSORS_ONLN);
-  assert(online_cpus > 0 && online_cpus <= processor_count(), "sanity check");
-  return online_cpus;
+  cpu_set_t cpus;  // can represent at most 1024 (CPU_SETSIZE) processors
+  int cpus_size = sizeof(cpu_set_t);
+  int cpu_count = 0;
+
+  // pid 0 means the current thread - which we have to assume represents the process
+  if (sched_getaffinity(0, cpus_size, &cpus) == 0) {
+    cpu_count = os_cpu_count(&cpus);
+    if (PrintActiveCpus) {
+      tty->print_cr("active_processor_count: sched_getaffinity processor count: %d", cpu_count);
+    }
+  }
+  else {
+    cpu_count = ::sysconf(_SC_NPROCESSORS_ONLN);
+    warning("sched_getaffinity failed (%s)- using online processor count (%d) "
+            "which may exceed available processors", strerror(errno), cpu_count);
+  }
+
+  assert(cpu_count > 0 && cpu_count <= processor_count(), "sanity check");
+  return cpu_count;
 }
 
 void os::set_native_thread_name(const char *name) {

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/runtime/os/AvailableProcessors.java	Thu Apr 27 05:04:03 2017 +0100
@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+import java.io.File;
+import com.oracle.java.testlibrary.ProcessTools;
+import com.oracle.java.testlibrary.OutputAnalyzer;
+import java.util.ArrayList;
+
+/*
+ * @test
+ * @bug 6515172
+ * @summary Check that availableProcessors reports the correct value when running in a cpuset on linux
+ * @requires os.family == "linux"
+ * @library /testlibrary
+ * @build com.oracle.java.testlibrary.*
+ * @run driver AvailableProcessors
+ */
+public class AvailableProcessors {
+
+    static final String SUCCESS_STRING = "Found expected processors: ";
+
+    public static void main(String[] args) throws Throwable {
+        if (args.length > 0)
+            checkProcessors(Integer.parseInt(args[0]));
+        else {
+            // run ourselves under different cpu configurations
+            // using the taskset command
+            String taskset;
+            final String taskset1 = "/bin/taskset";
+            final String taskset2 = "/usr/bin/taskset";
+            if (new File(taskset1).exists())
+                taskset = taskset1;
+            else if (new File(taskset2).exists())
+                taskset = taskset2;
+            else {
+                System.out.println("Skipping test: could not find taskset command");
+                return;
+            }
+
+            int available = Runtime.getRuntime().availableProcessors();
+
+            if (available == 1) {
+                System.out.println("Skipping test: only one processor available");
+                return;
+            }
+
+            // Get the java command we want to execute
+            // Enable logging for easier failure diagnosis
+            ProcessBuilder master =
+                    ProcessTools.createJavaProcessBuilder("-XX:+UnlockDiagnosticVMOptions",
+                                                          "-XX:+PrintActiveCpus",
+                                                          "AvailableProcessors");
+
+            int[] expected = new int[] { 1, available/2, available-1, available };
+
+            for (int i : expected) {
+                System.out.println("Testing for " + i + " processors ...");
+                int max = i - 1;
+                ArrayList<String> cmdline = new ArrayList<>(master.command());
+                // prepend taskset command
+                cmdline.add(0, "0-" + max);
+                cmdline.add(0, "-c");
+                cmdline.add(0, taskset);
+                // append expected processor count
+                cmdline.add(String.valueOf(i));
+                ProcessBuilder pb = new ProcessBuilder(cmdline);
+                System.out.println("Final command line: " +
+                                   ProcessTools.getCommandLine(pb));
+                OutputAnalyzer output = ProcessTools.executeProcess(pb);
+                output.shouldContain(SUCCESS_STRING);
+            }
+        }
+    }
+
+    static void checkProcessors(int expected) {
+        int available = Runtime.getRuntime().availableProcessors();
+        if (available != expected)
+            throw new Error("Expected " + expected + " processors, but found "
+                            + available);
+        else
+            System.out.println(SUCCESS_STRING + available);
+    }
+}