The task stack could be deallocated later. 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: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
If that works and there are no other objection then I'm going to repost
the complete series.
kernel/fork.c | 76 ++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 63 insertions(+), 13 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 984f69d6f211f..aa17ed2a2afc7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -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;
+ if (!try_release_thread_stack_to_cache(tsk->stack_vm_area))
+ thread_stack_delayed_free(tsk);
- for (i = 0; i < NR_CACHED_STACKS; i++) {
- if (this_cpu_cmpxchg(cached_stacks[i], NULL,
- tsk->stack_vm_area) != NULL)
- continue;
-
- 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;
}
--
2.34.1
|