On 1/25/22 07:26, Sebastian Andrzej Siewior wrote:
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.
I don't understand what this bit is for or why the logic needs to be this complicated. Can you set ->stack to NULL if and only if you freed it early?
+static void free_thread_stack(struct task_struct *tsk, bool cache_only)
This is messy. Please clean it up for real:
static bool try_release_thread_stack_to_cache(struct vm_struct *vm)
{
for (...) try to put it in this slot;
}
And the callers can do things like:
if (try_release_thread_stack_to_cache(...))
return;
/* need to free for real */
free it or delayed-free it.
--Andy
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