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

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Message-ID: <ad55978e-4356-494e-8cda-9e6487344768@kernel.dk>
Date: Fri, 7 Feb 2025 10:46:08 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Thomas Gleixner <tglx@...utronix.de>, 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 2/6/25 2:48 PM, Thomas Gleixner wrote:
> 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.

Sure I don't disagree - this is just my way of attempting to be nice and
suggest alternatives, as I don't like this patch.

>> 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.

Yep exactly, this is the way. Thanks!

-- 
Jens Axboe