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SubjectRe: [PATCH 3/4] kernel/fork: switch vmapped stack callation to __vmalloc_area()
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# 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 <khlebnikov@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);
>

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