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Message-ID: <1df49d97-df0e-4471-9e40-a850b758d981@colorfullife.com>
Date: Fri, 27 Dec 2024 19:39:06 +0100
From: Manfred Spraul <manfred@...orfullife.com>
To: Oleg Nesterov <oleg@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
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Subject: Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping
processes during pipe read/write
Hi,
(I had to remove many cc, my mail server rejected to send)
On 12/26/24 9:11 PM, Oleg Nesterov wrote:
> I mostly agree, see my reply to this patch, but...
>
> On 12/26, Linus Torvalds wrote:
>> If the optimization is correct, there is no point to having a config option.
>>
>> If the optimization is incorrect, there is no point to having the code.
>>
>> Either way, there's no way we'd ever have a config option for this.
> Agreed,
>
>>> + if (was_full && pipe_check_wq_has_sleeper(&pipe->wr_wait))
>>> wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
>> End result: you need to explain why the race cannot exist.
> Agreed,
>
>> And I think your patch is simply buggy
> Agreed again, but probably my reasoning was wrong,
>
>> But now waiters use "wait_event_interruptible_exclusive()" explicitly
>> outside the pipe mutex, so the waiters and wakers aren't actually
>> serialized.
> This is what I don't understand... Could you spell ?
> I _think_ that
>
> wait_event_whatever(WQ, CONDITION);
> vs
>
> CONDITION = 1;
> if (wq_has_sleeper(WQ))
> wake_up_xxx(WQ, ...);
>
> is fine.
This pattern is documented in wait.h:
https://elixir.bootlin.com/linux/v6.12.6/source/include/linux/wait.h#L96
Thus if there an issue, then the documentation should be updated.
> Both wq_has_sleeper() and wait_event_whatever()->prepare_to_wait_event()
> have the necessary barriers to serialize the waiters and wakers.
>
> Damn I am sure I missed something ;)
Actually:
Does this work universally? I.e. can we add the optimization into
__wake_up()?
But I do not understand this comment (from 2.6.0)
https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/kernel/fork.c?h=v2.6.0&id=e220fdf7a39b54a758f4102bdd9d0d5706aa32a7
> /* * Note: we use "set_current_state()" _after_ the wait-queue add, *
> because we need a memory barrier there on SMP, so that any *
> wake-function that tests for the wait-queue being active * will be
> guaranteed to see waitqueue addition _or_ subsequent * tests in this
> thread will see the wakeup having taken place. * * The spin_unlock()
> itself is semi-permeable and only protects * one way (it only protects
> stuff inside the critical region and * stops them from bleeding out -
> it would still allow subsequent * loads to move into the the critical
> region). */ voidprepare_to_wait
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/prepare_to_wait>(wait_queue_head_t
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/wait_queue_head_t>*q,wait_queue_t
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/wait_queue_t>*wait
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/wait>,intstate) {
> unsignedlongflags; wait
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/wait>->flags&=~WQ_FLAG_EXCLUSIVE
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/WQ_FLAG_EXCLUSIVE>;
> spin_lock_irqsave
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/spin_lock_irqsave>(&q->lock,flags);
> if(list_empty
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/list_empty>(&wait
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/wait>->task_list
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/task_list>))
> __add_wait_queue
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/__add_wait_queue>(q,wait
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/wait>);
> set_current_state
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/set_current_state>(state);
> spin_unlock_irqrestore
> <https://elixir.bootlin.com/linux/v2.6.0/C/ident/spin_unlock_irqrestore>(&q->lock,flags);
> }
set_current_state() now uses smp_store_mb(), which is a memory barrier
_after_ the store. Thus I do not see what enforces that the store
happens before the store for the __add_wait_queue().
--
Manfred
View attachment "0001-__wake_up_common_lock-Optimize-for-empty-wake-queues.patch" of type "text/x-patch" (2525 bytes)
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