rt: Fix scalability problem with big stacks on 32 bit

src/rt/rust_sched_loop.cpp

@@ -29,6 +29,8 @@ rust_sched_loop::rust_sched_loop(rust_scheduler *sched, int id, bool killed) :
     should_exit(false),
     cached_c_stack(NULL),
     extra_c_stack(NULL),
+    cached_big_stack(NULL),
+    extra_big_stack(NULL),
     dead_task(NULL),
     killed(killed),
     pump_signal(NULL),
@@ -263,6 +265,11 @@ rust_sched_loop::run_single_turn() {
             destroy_exchange_stack(kernel->region(), cached_c_stack);
             cached_c_stack = NULL;
         }
+        assert(!extra_big_stack);
+        if (cached_big_stack) {
+            destroy_exchange_stack(kernel->region(), cached_big_stack);
+            cached_big_stack = NULL;
+        }
 
         sched->release_task_thread();
         return sched_loop_state_exit;
@@ -392,6 +399,13 @@ rust_sched_loop::prepare_c_stack(rust_task *task) {
         cached_c_stack = create_exchange_stack(kernel->region(),
                                                C_STACK_SIZE);
     }
+    assert(!extra_big_stack);
+    if (!cached_big_stack) {
+        cached_big_stack = create_exchange_stack(kernel->region(),
+                                                 C_STACK_SIZE +
+                                                 (C_STACK_SIZE * 2));
+        cached_big_stack->is_big = 1;
+    }
 }
 
 void
@@ -400,6 +414,10 @@ rust_sched_loop::unprepare_c_stack() {
         destroy_exchange_stack(kernel->region(), extra_c_stack);
         extra_c_stack = NULL;
     }
+    if (extra_big_stack) {
+        destroy_exchange_stack(kernel->region(), extra_big_stack);
+        extra_big_stack = NULL;
+    }
 }
 
 //

src/rt/rust_sched_loop.h

@@ -67,6 +67,8 @@ private:
 
     stk_seg *cached_c_stack;
     stk_seg *extra_c_stack;
+    stk_seg *cached_big_stack;
+    stk_seg *extra_big_stack;
 
     rust_task_list running_tasks;
     rust_task_list blocked_tasks;
@@ -147,6 +149,10 @@ public:
     stk_seg *borrow_c_stack();
     void return_c_stack(stk_seg *stack);
 
+    // Called by tasks when they need a big stack
+    stk_seg *borrow_big_stack();
+    void return_big_stack(stk_seg *stack);
+
     int get_id() { return this->id; }
 };
 
@@ -202,6 +208,32 @@ rust_sched_loop::return_c_stack(stk_seg *stack) {
     }
 }
 
+// NB: Runs on the Rust stack. Might return NULL!
+inline stk_seg *
+rust_sched_loop::borrow_big_stack() {
+    assert(cached_big_stack);
+    stk_seg *your_stack;
+    if (extra_big_stack) {
+        your_stack = extra_big_stack;
+        extra_big_stack = NULL;
+    } else {
+        your_stack = cached_big_stack;
+        cached_big_stack = NULL;
+    }
+    return your_stack;
+}
+
+// NB: Runs on the Rust stack
+inline void
+rust_sched_loop::return_big_stack(stk_seg *stack) {
+    assert(!extra_big_stack);
+    assert(stack);
+    if (!cached_big_stack)
+        cached_big_stack = stack;
+    else
+        extra_big_stack = stack;
+}
+
 // this is needed to appease the circular dependency gods
 #include "rust_task.h"
 

src/rt/rust_stack.cpp

@@ -13,6 +13,8 @@
 #include "vg/valgrind.h"
 #include "vg/memcheck.h"
 
+#include <cstdio>
+
 #ifdef _LP64
 const uintptr_t canary_value = 0xABCDABCDABCDABCD;
 #else
@@ -61,6 +63,7 @@ create_stack(memory_region *region, size_t sz) {
     stk_seg *stk = (stk_seg *)region->malloc(total_sz, "stack");
     memset(stk, 0, sizeof(stk_seg));
     stk->end = (uintptr_t) &stk->data[sz];
+    stk->is_big = 0;
     add_stack_canary(stk);
     register_valgrind_stack(stk);
     return stk;
@@ -78,6 +81,7 @@ create_exchange_stack(rust_exchange_alloc *exchange, size_t sz) {
     stk_seg *stk = (stk_seg *)exchange->malloc(total_sz);
     memset(stk, 0, sizeof(stk_seg));
     stk->end = (uintptr_t) &stk->data[sz];
+    stk->is_big = 0;
     add_stack_canary(stk);
     register_valgrind_stack(stk);
     return stk;

src/rt/rust_stack.h

@@ -22,9 +22,7 @@ struct stk_seg {
     stk_seg *next;
     uintptr_t end;
     unsigned int valgrind_id;
-#ifndef _LP64
-    uint32_t pad;
-#endif
+    uint8_t is_big;
 
     rust_task *task;
     uintptr_t canary;

src/rt/rust_task.cpp

@@ -53,7 +53,8 @@ rust_task::rust_task(rust_sched_loop *sched_loop, rust_task_state state,
     disallow_yield(0),
     c_stack(NULL),
     next_c_sp(0),
-    next_rust_sp(0)
+    next_rust_sp(0),
+    big_stack(NULL)
 {
     LOGPTR(sched_loop, "new task", (uintptr_t)this);
     DLOG(sched_loop, task, "sizeof(task) = %d (0x%x)",
@@ -556,13 +557,64 @@ rust_task::cleanup_after_turn() {
     // Delete any spare stack segments that were left
     // behind by calls to prev_stack
     assert(stk);
+
     while (stk->next) {
         stk_seg *new_next = stk->next->next;
-        free_stack(stk->next);
+
+        if (stk->next->is_big) {
+            assert (big_stack == stk->next);
+            sched_loop->return_big_stack(big_stack);
+            big_stack = NULL;
+        } else {
+            free_stack(stk->next);
+        }
+
         stk->next = new_next;
     }
 }
 
+// NB: Runs on the Rust stack. Returns true if we successfully allocated the big
+// stack and false otherwise.
+bool
+rust_task::new_big_stack() {
+    // If we have a cached big stack segment, use it.
+    if (big_stack) {
+        // Check to see if we're already on the big stack.
+        stk_seg *ss = stk;
+        while (ss != NULL) {
+            if (ss == big_stack)
+                return false;
+            ss = ss->prev;
+        }
+
+        // Unlink the big stack.
+        if (big_stack->next)
+            big_stack->next->prev = big_stack->prev;
+        if (big_stack->prev)
+            big_stack->prev->next = big_stack->next;
+    } else {
+        stk_seg *borrowed_big_stack = sched_loop->borrow_big_stack();
+        if (!borrowed_big_stack) {
+            dump_stacks();
+            abort();
+        } else {
+            big_stack = borrowed_big_stack;
+        }
+    }
+
+    big_stack->task = this;
+    big_stack->next = stk->next;
+    if (big_stack->next)
+        big_stack->next->prev = big_stack;
+    big_stack->prev = stk;
+    if (stk)
+        stk->next = big_stack;
+
+    stk = big_stack;
+
+    return true;
+}
+
 static bool
 sp_in_stk_seg(uintptr_t sp, stk_seg *stk) {
     // Not positive these bounds for sp are correct.  I think that the first
@@ -602,9 +654,16 @@ rust_task::delete_all_stacks() {
     assert(stk->next == NULL);
     while (stk != NULL) {
         stk_seg *prev = stk->prev;
-        free_stack(stk);
+
+        if (stk->is_big)
+            sched_loop->return_big_stack(stk);
+        else
+            free_stack(stk);
+
         stk = prev;
     }
+
+    big_stack = NULL;
 }
 
 /*

src/rt/rust_task.h

@@ -133,6 +133,9 @@
 #define RZ_BSD_32   (1024*20)
 #define RZ_BSD_64   (1024*20)
 
+// The threshold beyond which we switch to the C stack.
+#define STACK_THRESHOLD (1024 * 1024)
+
 #ifdef __linux__
 #ifdef __i386__
 #define RED_ZONE_SIZE RZ_LINUX_32
@@ -263,9 +266,13 @@ private:
     uintptr_t next_c_sp;
     uintptr_t next_rust_sp;
 
+    // The big stack.
+    stk_seg *big_stack;
+
     // Called when the atomic refcount reaches zero
     void delete_this();
 
+    bool new_big_stack();
     void new_stack_fast(size_t requested_sz);
     void new_stack(size_t requested_sz);
     void free_stack(stk_seg *stk);
@@ -284,6 +291,8 @@ private:
                        char const *file,
                        size_t line);
 
+    void dump_stacks();
+
     friend void task_start_wrapper(spawn_args *a);
     friend void cleanup_task(cleanup_args *a);
     friend void reset_stack_limit_on_c_stack(reset_args *a);
@@ -568,6 +577,11 @@ rust_task::new_stack_fast(size_t requested_sz) {
     // The minimum stack size, in bytes, of a Rust stack, excluding red zone
     size_t min_sz = sched_loop->min_stack_size;
 
+    if (requested_sz > STACK_THRESHOLD) {
+        if (new_big_stack())
+            return;
+    }
+
     // Try to reuse an existing stack segment
     if (stk != NULL && stk->next != NULL) {
         size_t next_sz = user_stack_size(stk->next);