Messages in this thread | | | Date | Fri, 11 Feb 2022 15:55:01 -0800 | Subject | Re: [PATCH 7/8] kernel/fork: Only cache the VMAP stack in finish_task_switch(). | From | Andy Lutomirski <> |
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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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