Re: [PATCH 3/4] kernel/fork: switch vmapped stack callation to __vmalloc_area()

[Konstantin Khlebnikov]

# stress-ng --clone 100 -t 10s --metrics-brief
at 32-core machine shows boost 35000 -> 36000 bogo ops

Patch 4/4 is a kind of RFC.
Actually per-cpu cache of preallocated stacks works faster than buddy allocator 
thus
performance boots for it happens only at completely insane rate of clones.

On 23.01.2018 13:55, Konstantin Khlebnikov wrote:

This gives as pointer vm_struct without calling find_vm_area().

And fix comment about that task holds cache of vm area: this cache used
for retrieving actual stack pages, freeing is done by vfree_deferred().

Signed-off-by: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
---
  kernel/fork.c |   37 +++++++++++++++----------------------
  1 file changed, 15 insertions(+), 22 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 2295fc69717f..457c9151f3c8 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -204,39 +204,32 @@ static int free_vm_stack_cache(unsigned int cpu)
  static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int 
node)
  {
  #ifdef CONFIG_VMAP_STACK
-       void *stack;
+       struct vm_struct *stack;
        int i;

for (i = 0; i < NR_CACHED_STACKS; i++) {

-               struct vm_struct *s;
-
-               s = this_cpu_xchg(cached_stacks[i], NULL);
-
-               if (!s)
+               stack = this_cpu_xchg(cached_stacks[i], NULL);
+               if (!stack)
                        continue;

#ifdef CONFIG_DEBUG_KMEMLEAK

                /* Clear stale pointers from reused stack. */
-               memset(s->addr, 0, THREAD_SIZE);
+               memset(stack->addr, 0, THREAD_SIZE);
  #endif
-               tsk->stack_vm_area = s;
-               return s->addr;
+               tsk->stack_vm_area = stack;
+               return stack->addr;
        }

- stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,

-                                    VMALLOC_START, VMALLOC_END,
-                                    THREADINFO_GFP,
-                                    PAGE_KERNEL,
-                                    0, node, __builtin_return_address(0));
+       stack = __vmalloc_area(THREAD_SIZE, THREAD_ALIGN,
+                              VMALLOC_START, VMALLOC_END,
+                              THREADINFO_GFP, PAGE_KERNEL,
+                              0, node, __builtin_return_address(0));
+       if (unlikely(!stack))
+               return NULL;

- /*

-        * We can't call find_vm_area() in interrupt context, and
-        * free_thread_stack() can be called in interrupt context,
-        * so cache the vm_struct.
-        */
-       if (stack)
-               tsk->stack_vm_area = find_vm_area(stack);
-       return stack;
+       /* Cache the vm_struct for stack to page conversions. */
+       tsk->stack_vm_area = stack;
+       return stack->addr;
  #else
        struct page *page = alloc_pages_node(node, THREADINFO_GFP,
                                             THREAD_SIZE_ORDER);