rt: Make the initial segment of the main task's stack 1MB

This is a trick to fool microbenchmarks. Closes #1681

src/rt/rust.cpp

@@ -75,6 +75,8 @@ command_line_args : public kernel_owned<command_line_args>
 
 int check_claims = 0;
 
+const size_t MAIN_STACK_SIZE = 1024*1024;
+
 extern "C" CDECL int
 rust_start(uintptr_t main_fn, int argc, char **argv, void* crate_map) {
 
@@ -85,7 +87,7 @@ rust_start(uintptr_t main_fn, int argc, char **argv, void* crate_map) {
 
     rust_srv *srv = new rust_srv(env);
     rust_kernel *kernel = new rust_kernel(srv, env->num_sched_threads);
-    rust_task_id root_id = kernel->create_task(NULL, "main");
+    rust_task_id root_id = kernel->create_task(NULL, "main", MAIN_STACK_SIZE);
     rust_task *root_task = kernel->get_task_by_id(root_id);
     I(kernel, root_task != NULL);
     rust_scheduler *sched = root_task->sched;

src/rt/rust_kernel.cpp

@@ -151,15 +151,21 @@ rust_kernel::fail() {
 }
 
 rust_task_id
-rust_kernel::create_task(rust_task *spawner, const char *name) {
+rust_kernel::create_task(rust_task *spawner, const char *name,
+                         size_t init_stack_sz) {
     scoped_lock with(_kernel_lock);
     rust_scheduler *thread = threads[isaac_rand(&rctx) % num_threads];
-    rust_task *t = thread->create_task(spawner, name);
+    rust_task *t = thread->create_task(spawner, name, init_stack_sz);
     t->user.id = max_id++;
     task_table.put(t->user.id, t);
     return t->user.id;
 }
 
+rust_task_id
+rust_kernel::create_task(rust_task *spawner, const char *name) {
+    return create_task(spawner, name, env->min_stack_size);
+}
+
 rust_task *
 rust_kernel::get_task_by_id(rust_task_id id) {
     scoped_lock with(_kernel_lock);

src/rt/rust_kernel.h

@@ -66,7 +66,9 @@ public:
     void win32_require(LPCTSTR fn, BOOL ok);
 #endif
 
-    rust_task_id create_task(rust_task *spawner, const char *name);
+    rust_task_id create_task(rust_task *spawner, const char *name,
+			     size_t init_stack_size);
+    rust_task_id create_task(rust_task * spawner, const char *name);
     rust_task *get_task_by_id(rust_task_id id);
     void release_task_id(rust_task_id tid);
     void set_exit_status(int code);

src/rt/rust_scheduler.cpp

@@ -333,10 +333,11 @@ rust_scheduler::get_cache() {
 }
 
 rust_task *
-rust_scheduler::create_task(rust_task *spawner, const char *name) {
+rust_scheduler::create_task(rust_task *spawner, const char *name,
+                            size_t init_stack_sz) {
     rust_task *task =
         new (this->kernel, "rust_task")
-        rust_task (this, &newborn_tasks, spawner, name);
+        rust_task (this, &newborn_tasks, spawner, name, init_stack_sz);
     DLOG(this, task, "created task: " PTR ", spawner: %s, name: %s",
                         task, spawner ? spawner->name : "null", name);
     if(spawner) {

src/rt/rust_scheduler.h

@@ -112,7 +112,8 @@ struct rust_scheduler : public kernel_owned<rust_scheduler>,
 
     void kill_all_tasks();
 
-    rust_task *create_task(rust_task *spawner, const char *name);
+    rust_task *create_task(rust_task *spawner, const char *name,
+                           size_t init_stack_sz);
 
     virtual void run();
 

src/rt/rust_task.cpp

@@ -239,7 +239,8 @@ del_stk(rust_task *task, stk_seg *stk)
 
 // Tasks
 rust_task::rust_task(rust_scheduler *sched, rust_task_list *state,
-                     rust_task *spawner, const char *name) :
+                     rust_task *spawner, const char *name,
+                     size_t init_stack_sz) :
     ref_count(1),
     stk(NULL),
     runtime_sp(0),
@@ -271,7 +272,7 @@ rust_task::rust_task(rust_scheduler *sched, rust_task_list *state,
 
     user.notify_enabled = 0;
 
-    stk = new_stk(sched, this, 0);
+    stk = new_stk(sched, this, init_stack_sz);
     user.rust_sp = stk->end;
     if (supervisor) {
         supervisor->ref();

src/rt/rust_task.h

@@ -133,7 +133,8 @@ rust_task : public kernel_owned<rust_task>, rust_cond
     rust_task(rust_scheduler *sched,
               rust_task_list *state,
               rust_task *spawner,
-              const char *name);
+              const char *name,
+              size_t init_stack_sz);
 
     ~rust_task();