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Date: Thu, 18 Nov 2021 15:34:51 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Andy Lutomirski <luto@...nel.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 7/8] kernel/fork: Only cache the VMAP stack in finish_task_switch().
The task stack could be deallocated later in delayed_put_task_struct().
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.
If the free/caching is RCU-delayed then a new stack has to be allocated
because the cache is filled in batches of which only two stacks, out of
many, are recycled.
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.
Introduce put_task_stack_sched() which is invoked from the
finish_task_switch() and only caches the VMAP stack. If the cache is
full or !CONFIG_VMAP_STACK is used than the stack is freed from
delayed_put_task_struct(). In the VMAP case this is another opportunity
to fill the cache.
The stack is finally released in delayed_put_task_struct() which means
that a valid stack reference can be held during its invocation. As such
there can be made no assumption whether the task_struct::stack pointer
can be freed if non-NULL.
Set the lowest bit of task_struct::stack if the stack was released via
put_task_stack_sched() and needs a final free in
delayed_put_task_struct(). If the bit is missing then a reference is
held and put_task_stack() will release it.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
include/linux/sched/task_stack.h | 8 +++++
kernel/exit.c | 1 +
kernel/fork.c | 60 ++++++++++++++++++++++++++------
kernel/sched/core.c | 7 ++--
4 files changed, 64 insertions(+), 12 deletions(-)
diff --git a/include/linux/sched/task_stack.h b/include/linux/sched/task_stack.h
index 892562ebbd3aa..ccd1336aa7f42 100644
--- a/include/linux/sched/task_stack.h
+++ b/include/linux/sched/task_stack.h
@@ -70,6 +70,7 @@ static inline void *try_get_task_stack(struct task_struct *tsk)
}
extern void put_task_stack(struct task_struct *tsk);
+extern void put_task_stack_sched(struct task_struct *tsk);
#else
static inline void *try_get_task_stack(struct task_struct *tsk)
{
@@ -77,6 +78,13 @@ static inline void *try_get_task_stack(struct task_struct *tsk)
}
static inline void put_task_stack(struct task_struct *tsk) {}
+static inline void put_task_stack_sched(struct task_struct *tsk) {}
+#endif
+
+#ifdef CONFIG_ARCH_THREAD_STACK_ALLOCATOR
+static inline void task_stack_cleanup(struct task_struct *tsk) {}
+#else
+extern void task_stack_cleanup(struct task_struct *tsk);
#endif
void exit_task_stack_account(struct task_struct *tsk);
diff --git a/kernel/exit.c b/kernel/exit.c
index 41236da175d56..383a56795e82a 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -171,6 +171,7 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
kprobe_flush_task(tsk);
perf_event_delayed_put(tsk);
trace_sched_process_free(tsk);
+ task_stack_cleanup(tsk);
put_task_struct(tsk);
}
diff --git a/kernel/fork.c b/kernel/fork.c
index 257cd048359fd..0aa079e56e3b4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -178,6 +178,16 @@ static inline void free_task_struct(struct task_struct *tsk)
#ifndef CONFIG_ARCH_THREAD_STACK_ALLOCATOR
+#define THREAD_STACK_DELAYED_FREE 1UL
+
+static void thread_stack_mark_delayed_free(struct task_struct *tsk)
+{
+ unsigned long val = (unsigned long)tsk->stack;
+
+ val |= THREAD_STACK_DELAYED_FREE;
+ WRITE_ONCE(tsk->stack, (void *)val);
+}
+
/*
* Allocate pages if THREAD_SIZE is >= PAGE_SIZE, otherwise use a
* kmemcache based allocator.
@@ -293,7 +303,7 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
return 0;
}
-static void free_thread_stack(struct task_struct *tsk)
+static void free_thread_stack(struct task_struct *tsk, bool cache_only)
{
int i;
@@ -306,7 +316,12 @@ static void free_thread_stack(struct task_struct *tsk)
tsk->stack_vm_area = NULL;
return;
}
- vfree_atomic(tsk->stack);
+ if (cache_only) {
+ thread_stack_mark_delayed_free(tsk);
+ return;
+ }
+
+ vfree(tsk->stack);
tsk->stack = NULL;
tsk->stack_vm_area = NULL;
}
@@ -325,8 +340,12 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
return -ENOMEM;
}
-static void free_thread_stack(struct task_struct *tsk)
+static void free_thread_stack(struct task_struct *tsk, bool cache_only)
{
+ if (cache_only) {
+ thread_stack_mark_delayed_free(tsk);
+ return;
+ }
__free_pages(virt_to_page(tsk->stack), THREAD_SIZE_ORDER);
tsk->stack = NULL;
}
@@ -345,8 +364,12 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
return stack ? 0 : -ENOMEM;
}
-static void free_thread_stack(struct task_struct *tsk)
+static void free_thread_stack(struct task_struct *tsk, bool cache_only)
{
+ if (cache_only) {
+ thread_stack_mark_delayed_free(tsk);
+ return;
+ }
kmem_cache_free(thread_stack_cache, tsk->stack);
tsk->stack = NULL;
}
@@ -360,8 +383,19 @@ void thread_stack_cache_init(void)
}
# endif /* THREAD_SIZE >= PAGE_SIZE || defined(CONFIG_VMAP_STACK) */
-#else /* CONFIG_ARCH_THREAD_STACK_ALLOCATOR */
+void task_stack_cleanup(struct task_struct *tsk)
+{
+ unsigned long val = (unsigned long)tsk->stack;
+
+ if (!(val & THREAD_STACK_DELAYED_FREE))
+ return;
+
+ WRITE_ONCE(tsk->stack, (void *)(val & ~THREAD_STACK_DELAYED_FREE));
+ free_thread_stack(tsk, false);
+}
+
+#else /* CONFIG_ARCH_THREAD_STACK_ALLOCATOR */
static int alloc_thread_stack_node(struct task_struct *tsk, int node)
{
unsigned long *stack;
@@ -461,19 +495,25 @@ void exit_task_stack_account(struct task_struct *tsk)
}
}
-static void release_task_stack(struct task_struct *tsk)
+static void release_task_stack(struct task_struct *tsk, bool cache_only)
{
if (WARN_ON(READ_ONCE(tsk->__state) != TASK_DEAD))
return; /* Better to leak the stack than to free prematurely */
- free_thread_stack(tsk);
+ free_thread_stack(tsk, cache_only);
}
#ifdef CONFIG_THREAD_INFO_IN_TASK
void put_task_stack(struct task_struct *tsk)
{
if (refcount_dec_and_test(&tsk->stack_refcount))
- release_task_stack(tsk);
+ release_task_stack(tsk, false);
+}
+
+void put_task_stack_sched(struct task_struct *tsk)
+{
+ if (refcount_dec_and_test(&tsk->stack_refcount))
+ release_task_stack(tsk, true);
}
#endif
@@ -487,7 +527,7 @@ void free_task(struct task_struct *tsk)
* The task is finally done with both the stack and thread_info,
* so free both.
*/
- release_task_stack(tsk);
+ release_task_stack(tsk, false);
#else
/*
* If the task had a separate stack allocation, it should be gone
@@ -989,7 +1029,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
free_stack:
exit_task_stack_account(tsk);
- free_thread_stack(tsk);
+ free_thread_stack(tsk, false);
free_tsk:
free_task_struct(tsk);
return NULL;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3c9b0fda64ac0..435f633f6f365 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4880,8 +4880,11 @@ static struct rq *finish_task_switch(struct task_struct *prev)
if (prev->sched_class->task_dead)
prev->sched_class->task_dead(prev);
- /* Task is done with its stack. */
- put_task_stack(prev);
+ /*
+ * Cache only the VMAP stack. The final deallocation is in
+ * delayed_put_task_struct.
+ */
+ put_task_stack_sched(prev);
put_task_struct_rcu_user(prev);
}
--
2.33.1
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