Re: [PATCH v4 1/4] mm: reduce the rcu lock duration

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/5/31 21:05, Matthew Wilcox wrote:
> On Tue, May 31, 2022 at 01:58:31PM +0100, Matthew Wilcox wrote:
>> On Mon, May 30, 2022 at 07:30:13PM +0800, Miaohe Lin wrote:
>>> Commit 3268c63eded4 ("mm: fix move/migrate_pages() race on task struct")
>>> extends the period of the rcu_read_lock until after the permissions checks
>>> are done to prevent the task pointed to from changing from under us. But
>>> the task_struct refcount is also taken at that time, the reference to task
>>> is guaranteed to be stable. So it's unnecessary to extend the period of
>>> the rcu_read_lock. Release the rcu lock after task refcount is successfully
>>> grabbed to reduce the rcu holding time.
>>
>> But why bother?  You know the RCU read lock isn't a "real" lock, right?
> 
> Looking over this code some more, I think this may harm performance.
> ptrace_may_access() itself takes the rcu_read_lock().  So we currently
> have:
> 
> rcu_read_lock()
> rcu_read_lock();
> rcu_read_unlock();
> rcu_read_unlock();

More precisely, we currently have:

rcu_read_lock()
task_lock()
rcu_read_lock();
rcu_read_unlock();
task_unlock()
rcu_read_unlock();

> 
> In at least one RCU configuration, rcu_read_lock() maps to
> preempt_disable().  Nested preempt_disable() just bump a counter, while
> that counter reaching zero incurs some actual work.  So nested
> rcu_read_lock() can be better than sequential lock/unlock/lock/unlock.

In this case, I agree with you.
But when task_lock is heavily contented, it might take a long time. So in this
case, I think it's better to do the sequential rcu_lock+unlock to avoid long
rcu lock duration. Or am I miss something?

> 
> This needs far better justification.

Thanks!

> 
> .
>