On Sat 04-02-17 00:30:05, Hoeun Ryu wrote:
> Using virtually mapped stack, kernel stacks are allocated via vmalloc.
> In the current implementation, two stacks per cpu can be cached when
> tasks are freed and the cached stacks are used again in task duplications.
> but the array for the cached stacks is statically allocated by per-cpu api.
> In this new implementation, the array for the cached stacks are dynamically
> allocted and freed by cpu hotplug callbacks and the cached stacks are freed
> when cpu is down. setup for cpu hotplug is established in fork_init().
Why do we want this? I can see that the follow up patch makes the number
configurable but the changelog doesn't describe the motivation for that.
Which workload would benefit from a higher value?
> Signed-off-by: Hoeun Ryu <hoeun....@gmail.com>
> ---
> kernel/fork.c | 81
> ++++++++++++++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 64 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 61284d8..54421a9 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -167,26 +167,71 @@ void __weak arch_release_thread_stack(unsigned long
> *stack)
> * 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]);
> +
> +struct vm_stack_cache {
> + struct vm_struct **vm_stacks;
> + int nr;
> + int cur;
> +};
> +
> +static DEFINE_PER_CPU(struct vm_stack_cache, vm_stacks);
> +
> +static int alloc_vm_stack_cache(unsigned int cpu)
> +{
> + struct vm_stack_cache *vm_stack_cache = &per_cpu(vm_stacks, cpu);
> + struct vm_struct **vm_stacks = vm_stack_cache->vm_stacks;
> + int i;
> +
> + /* if free_vm_stack_cache() didn't free it */
> + if (!vm_stacks) {
> + vm_stacks =
> + vzalloc(sizeof(struct vm_struct *) * NR_CACHED_STACKS);
> + if (!vm_stacks)
> + return -ENOMEM;
> + }
> +
> + vm_stack_cache->vm_stacks = vm_stacks;
> + vm_stack_cache->cur = 0;
> + vm_stack_cache->nr = 0;
> +
> + return 0;
> +}
> +
> +static int free_vm_stack_cache(unsigned int cpu)
> +{
> + struct vm_stack_cache *vm_stack_cache = &per_cpu(vm_stacks, cpu);
> + struct vm_struct **vm_stacks = vm_stack_cache->vm_stacks;
> + int i;
> +
> + for (i = 0; i < vm_stack_cache->nr; i++) {
> + vfree(vm_stacks[i]->addr);
> + vm_stacks[i] = NULL;
> + }
> +
> + vm_stack_cache->nr = 0;
> + vm_stack_cache->cur = 0;
> + /* do not free vm_stack[cpu]->vm_stacks itself, reused in allocation */
> +
> + return 0;
> +}
> +
> #endif
>
> static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int
> node)
> {
> #ifdef CONFIG_VMAP_STACK
> + struct vm_stack_cache *vm_stack_cache =
> + &per_cpu(vm_stacks, smp_processor_id());
> + struct vm_struct **vm_stacks = vm_stack_cache->vm_stacks;
> 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;
> + if (vm_stack_cache->cur > 0) {
> + struct vm_struct *vm_stack = vm_stacks[--vm_stack_cache->cur];
> + tsk->stack_vm_area = vm_stack;
> local_irq_enable();
> - return s->addr;
> +
> + return vm_stack->addr;
> }
> local_irq_enable();
>
> @@ -216,15 +261,14 @@ static inline void free_thread_stack(struct task_struct
> *tsk)
> {
> #ifdef CONFIG_VMAP_STACK
> if (task_stack_vm_area(tsk)) {
> + struct vm_stack_cache *vm_stack_cache =
> + &per_cpu(vm_stacks, smp_processor_id());
> + struct vm_struct **vm_stacks = vm_stack_cache->vm_stacks;
> 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);
> + if (vm_stack_cache->cur < vm_stack_cache->nr) {
> + vm_stacks[vm_stack_cache->cur++] = tsk->stack_vm_area;
> local_irq_restore(flags);
> return;
> }
> @@ -456,6 +500,9 @@ void __init fork_init(void)
> for (i = 0; i < UCOUNT_COUNTS; i++) {
> init_user_ns.ucount_max[i] = max_threads/2;
> }
> +
> + cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "vm_stack_cache",
> + alloc_vm_stack_cache, free_vm_stack_cache);
> }
>
> int __weak arch_dup_task_struct(struct task_struct *dst,
> --
> 2.7.4
>
--
Michal Hocko
SUSE Labs
