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

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Message-ID: <4f317ca3-0ffd-4ee0-9da6-8b80a6366d0f@kernel.dk>
Date: Thu, 6 Feb 2025 13:05:48 -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 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.

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

-- 
Jens Axboe