linux-kernel - Re: [PATCH v2 05/12] io_uring: Use helper function hrtimer_update

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Message-ID: <87h656n3r6.ffs@tglx>
Date: Thu, 06 Feb 2025 22:48:45 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Jens Axboe <axboe@...nel.dk>, Nam Cao <namcao@...utronix.de>, Anna-Maria
 Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker
 <frederic@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/12] io_uring: Use helper function
 hrtimer_update_function()

On Thu, Feb 06 2025 at 13:05, Jens Axboe wrote:
> On 2/6/25 9:18 AM, Thomas Gleixner wrote:
>> On Wed, Feb 05 2025 at 11:45, Jens Axboe wrote:
>>> On 2/5/25 3:55 AM, Nam Cao wrote:
>>>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>>>> index ceacf6230e34..936f8b4106cf 100644
>>>> --- a/io_uring/io_uring.c
>>>> +++ b/io_uring/io_uring.c
>>>> @@ -2421,7 +2421,7 @@ static enum hrtimer_restart io_cqring_min_timer_wakeup(struct hrtimer *timer)
>>>>  			goto out_wake;
>>>>  	}
>>>>  
>>>> -	iowq->t.function = io_cqring_timer_wakeup;
>>>> +	hrtimer_update_function(&iowq->t, io_cqring_timer_wakeup);
>>>>  	hrtimer_set_expires(timer, iowq->timeout);
>>>>  	return HRTIMER_RESTART;
>>>>  out_wake:
>>>
>>> The timer is known expired here, it's inside the callback. Is this
>>> really necessary or useful?
>> 
>> It's not strictly required here, but in the end this allows to make the
>> .function member private, which prevents stupid code fiddling with it
>> without proper sanity checks in the debug case.
>
> While that makes sense, this is also a potentially hot path for min
> timeout usage, which with small timeouts for batch/latency reasons
> can be run tens of thousand times per second. Adding a lock and IRQ
> dance would be counter productive.

I fundamentally hate the fact that C does not enforce encapsulation in
the first place. :)

> How about we just add a comment on why this is fine, rather than

Comments are the worst of a solution as you know.

> slow down a case that's perfectly fine by wrapping it in something
> much more expensive than a simple memory write? Or perhaps have
> a basic helper to set it that doesn't do the unnecessary irq lock
> guard? That would still allow you to make .function private.

The right solution is to add a hotpath helper, which falls back to the
more expensive variant with some anyway expensive debug option, or make
the expensive part of hrtimer_update_function() depend on that debug
option in the first place and otherwise fall back to a simple store.

The latter is the right thing to do. Let me fix that up in mainline.

Thanks,

        tglx