Return error when no core is available

clone returns -EAGAIN when there is no vacant core.
In addition, clone tries to use the next vacant hyper-threading
core instead of trying to use next vacant hyper-threading core
of the next vacant physical core.

arch/x86/kernel/syscall.c

@@ -72,21 +72,31 @@ int obtain_clone_cpuid() {
  retry:
     /* Try to obtain next physical core */
     cpuid = cpuid_head;
-    cpuid_head += 4;
+
+    /* A hyper-threading core on the same physical core as
+       the parent process might be chosen. Use sched_setaffinity
+       if you want to skip that kind of busy physical core for
+       performance reason. */
+    cpuid_head += 1;
     if(cpuid_head >= cpu_info->ncpus) {
-        cpuid_head = ((cpuid_head % 4) + 1) % 4;
+        cpuid_head = 0;
     }
-    /* Don't use a physical core with a system process (e.g. MPI)
-       because using it degrades performance */
-    if((cpu_info->ncpus - 3 <= cpuid && cpuid <= cpu_info->ncpus - 1) ||
-       get_cpu_local_var(cpuid)->status != CPU_STATUS_IDLE) {
+
+    /* A hyper-threading core whose parent physical core has a
+       process on one of its hyper-threading core might
+       be chosen. Use sched_setaffinity if you want to skip that
+       kind of busy physical core for performance reason. */
+    if(get_cpu_local_var(cpuid)->status != CPU_STATUS_IDLE) {
         nretry++;
         if(nretry >= cpu_info->ncpus) {
-            panic("there is no cpu with empty runq\n");
+            cpuid = -1;
+            ihk_mc_spinlock_unlock_noirq(&cpuid_head_lock);
+            goto out;
         }
         goto retry; 
     }
     ihk_mc_spinlock_unlock_noirq(&cpuid_head_lock);
+ out:
     return cpuid;
 }
 

kernel/syscall.c

@@ -1290,6 +1290,9 @@ SYSCALL_DECLARE(clone)
 			 (unsigned long)ihk_mc_syscall_sp(ctx));
 
 	cpuid = obtain_clone_cpuid();
+    if (cpuid == -1) {
+        return -EAGAIN;
+    }
 
 	new = clone_process(cpu_local_var(current), ihk_mc_syscall_pc(ctx),
 	                    ihk_mc_syscall_arg1(ctx) ? ihk_mc_syscall_arg1(ctx) :