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

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Message-ID: <8d56b9d7-bb92-4c6e-ba8b-da3ec238943b@colorfullife.com>
Date: Sat, 28 Dec 2024 19:53:57 +0100
From: Manfred Spraul <manfred@...orfullife.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 WangYuli <wangyuli@...ontech.com>,
 linux-fsdevel <linux-fsdevel@...r.kernel.org>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 Christian Brauner <brauner@...nel.org>
Subject: Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping
 processes during pipe read/write

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