Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping processes during pipe read/write

[Manfred Spraul]

Hi Oleg,

On 12/28/24 5:32 PM, Oleg Nesterov wrote:
> On 12/28, Oleg Nesterov wrote:
> > If nothing else, consider
> >
> > 	int CONDITION;
> > 	wait_queue_head_t WQ;
> >
> > 	void wake(void)
> > 	{
> > 		CONDITION = 1;
> > 		wake_up(WQ);
> > 	}
> >
> > 	void wait(void)
> > 	{
> > 		DEFINE_WAIT_FUNC(entry, woken_wake_function);
> >
> > 		add_wait_queue(WQ, entry);
> > 		if (!CONDITION)
> > 			wait_woken(entry, ...);
> > 		remove_wait_queue(WQ, entry);
> > 	}
> >
> > this code is correct even if LOAD(CONDITION) can leak into the critical
> > section in add_wait_queue(), so CPU running wait() can actually do
> >
> > 		// add_wait_queue
> > 		spin_lock(WQ->lock);
> > 		LOAD(CONDITION);	// false!
> > 		list_add(entry, head);
> > 		spin_unlock(WQ->lock);
> >
> > 		if (!false)		// result of the LOAD above
> > 			wait_woken(entry, ...);
> >
> > Now suppose that another CPU executes wake() between LOAD(CONDITION)
> > and list_add(entry, head). With your patch wait() will miss the event.
> > The same for __pollwait(), I think...
> >
> > No?
> Even simpler,
>
> 	void wait(void)
> 	{
> 		DEFINE_WAIT(entry);
>
> 		__set_current_state(XXX);
> 		add_wait_queue(WQ, entry);
>
> 		if (!CONDITION)
> 			schedule();
>
> 		remove_wait_queue(WQ, entry);
> 		__set_current_state(TASK_RUNNING);
> 	}
>
> This code is ugly but currently correct unless I am totally confused.

It is a chance of the add_wait_queue() path, thus impact on all calls.

With (busybox) "find /sys /proc | grep aaaabbbccc", I've seen 16385 
wakeup calls with empty queue, and just 6 with an entry in the queue.

But on other workloads, the ratio was more something like 75% 
empty/25%with entries.

I.e.: We would have long discussions if the change only helps for some 
usecases, and might have negative impact on other use cases.


And: Your proposal is in conflict with

https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/kernel/fork.c?h=v2.6.0&id=e220fdf7a39b54a758f4102bdd9d0d5706aa32a7 


But I do not see the issue, the worst possible scenario should be something like:

	// add_wait_queue
		spin_lock(WQ->lock);
		LOAD(CONDITION);	// false!
		list_add(entry, head);
		STORE(current_state)
		spin_unlock(WQ->lock);


--

    Manfred