Commit e540bf31 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Thomas Gleixner
Browse files

fork: Only cache the VMAP stack in finish_task_switch() 



The task stack could be deallocated later, but for fork()/exec() kind of
workloads (say a shell script executing several commands) it is important
that the stack is released in finish_task_switch() so that in VMAP_STACK
case it can be cached and reused in the new task.

For PREEMPT_RT it would be good if the wake-up in vfree_atomic() could
be avoided in the scheduling path. Far worse are the other
free_thread_stack() implementations which invoke __free_pages()/
kmem_cache_free() with disabled preemption.

Cache the stack in free_thread_stack() in the VMAP_STACK case and
RCU-delay the free path otherwise. Free the stack in the RCU callback.
In the VMAP_STACK case this is another opportunity to fill the cache.

Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarAndy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20220217102406.3697941-8-bigeasy@linutronix.de
parent 1a03d3f1

kernel/fork.c

+63 −13
Original line number Diff line number Diff line
@@ -193,6 +193,41 @@ static inline void free_task_struct(struct task_struct *tsk) 
#define NR_CACHED_STACKS 2

static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);



struct vm_stack {

	struct rcu_head rcu;

	struct vm_struct *stack_vm_area;

};



static bool try_release_thread_stack_to_cache(struct vm_struct *vm)

{

	unsigned int i;



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

		if (this_cpu_cmpxchg(cached_stacks[i], NULL, vm) != NULL)

			continue;

		return true;

	}

	return false;

}



static void thread_stack_free_rcu(struct rcu_head *rh)

{

	struct vm_stack *vm_stack = container_of(rh, struct vm_stack, rcu);



	if (try_release_thread_stack_to_cache(vm_stack->stack_vm_area))

		return;



	vfree(vm_stack);

}



static void thread_stack_delayed_free(struct task_struct *tsk)

{

	struct vm_stack *vm_stack = tsk->stack;



	vm_stack->stack_vm_area = tsk->stack_vm_area;

	call_rcu(&vm_stack->rcu, thread_stack_free_rcu);

}



static int free_vm_stack_cache(unsigned int cpu)

{

	struct vm_struct **cached_vm_stacks = per_cpu_ptr(cached_stacks, cpu);
@@ -296,24 +331,27 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node) 


static void free_thread_stack(struct task_struct *tsk)

{

	int i;



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

		if (this_cpu_cmpxchg(cached_stacks[i], NULL,

				     tsk->stack_vm_area) != NULL)

			continue;

	if (!try_release_thread_stack_to_cache(tsk->stack_vm_area))

		thread_stack_delayed_free(tsk);



	tsk->stack = NULL;

	tsk->stack_vm_area = NULL;

		return;

	}

	vfree_atomic(tsk->stack);

	tsk->stack = NULL;

	tsk->stack_vm_area = NULL;

}



#  else /* !CONFIG_VMAP_STACK */



static void thread_stack_free_rcu(struct rcu_head *rh)

{

	__free_pages(virt_to_page(rh), THREAD_SIZE_ORDER);

}



static void thread_stack_delayed_free(struct task_struct *tsk)

{

	struct rcu_head *rh = tsk->stack;



	call_rcu(rh, thread_stack_free_rcu);

}



static int alloc_thread_stack_node(struct task_struct *tsk, int node)

{

	struct page *page = alloc_pages_node(node, THREADINFO_GFP,
@@ -328,7 +366,7 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node) 


static void free_thread_stack(struct task_struct *tsk)

{

	__free_pages(virt_to_page(tsk->stack), THREAD_SIZE_ORDER);

	thread_stack_delayed_free(tsk);

	tsk->stack = NULL;

}
@@ -337,6 +375,18 @@ static void free_thread_stack(struct task_struct *tsk) 


static struct kmem_cache *thread_stack_cache;



static void thread_stack_free_rcu(struct rcu_head *rh)

{

	kmem_cache_free(thread_stack_cache, rh);

}



static void thread_stack_delayed_free(struct task_struct *tsk)

{

	struct rcu_head *rh = tsk->stack;



	call_rcu(rh, thread_stack_free_rcu);

}



static int alloc_thread_stack_node(struct task_struct *tsk, int node)

{

	unsigned long *stack;
@@ -348,7 +398,7 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node) 


static void free_thread_stack(struct task_struct *tsk)

{

	kmem_cache_free(thread_stack_cache, tsk->stack);

	thread_stack_delayed_free(tsk);

	tsk->stack = NULL;

}