Re: [PATCH v2 05/12] io_uring: Use helper function hrtimer_update_function()

From: Jens Axboe
Date: Thu Feb 06 2025 - 15:06:00 EST


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