fork: Optimize task creation by caching two thread stacks per CPU if CONFIG_VMAP_STACK=y

vmalloc() is a bit slow, and pounding vmalloc()/vfree() will eventually
force a global TLB flush.

To reduce pressure on them, if CONFIG_VMAP_STACK=y, cache two thread
stacks per CPU.  This will let us quickly allocate a hopefully
cache-hot, TLB-hot stack under heavy forking workloads (shell script style).

On my silly pthread_create() benchmark, it saves about 2 µs per
pthread_create()+join() with CONFIG_VMAP_STACK=y.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jann Horn <jann@thejh.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/94811d8e3994b2e962f88866290017d498eb069c.1474003868.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/fork.c b/kernel/fork.c
index 5dd0a51..c060c7e 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -159,15 +159,41 @@ void __weak arch_release_thread_stack(unsigned long *stack)
  * kmemcache based allocator.
  */
 # if THREAD_SIZE >= PAGE_SIZE || defined(CONFIG_VMAP_STACK)
+
+#ifdef CONFIG_VMAP_STACK
+/*
+ * vmalloc() is a bit slow, and calling vfree() enough times will force a TLB
+ * flush.  Try to minimize the number of calls by caching stacks.
+ */
+#define NR_CACHED_STACKS 2
+static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);
+#endif
+
 static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 {
 #ifdef CONFIG_VMAP_STACK
-       void *stack = __vmalloc_node_range(THREAD_SIZE, THREAD_SIZE,
-                                          VMALLOC_START, VMALLOC_END,
-                                          THREADINFO_GFP | __GFP_HIGHMEM,
-                                          PAGE_KERNEL,
-                                          0, node,
-                                          __builtin_return_address(0));
+       void *stack;
+       int i;
+
+       local_irq_disable();
+       for (i = 0; i < NR_CACHED_STACKS; i++) {
+               struct vm_struct *s = this_cpu_read(cached_stacks[i]);
+
+               if (!s)
+                       continue;
+               this_cpu_write(cached_stacks[i], NULL);
+
+               tsk->stack_vm_area = s;
+               local_irq_enable();
+               return s->addr;
+       }
+       local_irq_enable();
+
+       stack = __vmalloc_node_range(THREAD_SIZE, THREAD_SIZE,
+                                    VMALLOC_START, VMALLOC_END,
+                                    THREADINFO_GFP | __GFP_HIGHMEM,
+                                    PAGE_KERNEL,
+                                    0, node, __builtin_return_address(0));
 
        /*
         * We can't call find_vm_area() in interrupt context, and
@@ -187,10 +213,28 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 
 static inline void free_thread_stack(struct task_struct *tsk)
 {
-       if (task_stack_vm_area(tsk))
+#ifdef CONFIG_VMAP_STACK
+       if (task_stack_vm_area(tsk)) {
+               unsigned long flags;
+               int i;
+
+               local_irq_save(flags);
+               for (i = 0; i < NR_CACHED_STACKS; i++) {
+                       if (this_cpu_read(cached_stacks[i]))
+                               continue;
+
+                       this_cpu_write(cached_stacks[i], tsk->stack_vm_area);
+                       local_irq_restore(flags);
+                       return;
+               }
+               local_irq_restore(flags);
+
                vfree(tsk->stack);
-       else
-               __free_pages(virt_to_page(tsk->stack), THREAD_SIZE_ORDER);
+               return;
+       }
+#endif
+
+       __free_pages(virt_to_page(tsk->stack), THREAD_SIZE_ORDER);
 }
 # else
 static struct kmem_cache *thread_stack_cache;