linux-kernel - Re: Query about timer wheel API

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Message-ID: <266befac-b781-4a37-9261-6d956cceb36f@oracle.com>
Date: Thu, 16 Jan 2025 00:48:56 +1100
From: imran.f.khan@...cle.com
To: Thomas Gleixner <tglx@...utronix.de>
Cc: john.stultz@...aro.org, sboyd@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: Query about timer wheel API

Hello Thomas,
Thank you so much for your feedback.
On 15/1/2025 9:32 pm, Thomas Gleixner wrote:
> On Mon, Dec 23 2024 at 11:14, imran f. khan wrote:
>> Could you kindly help me, regarding a query about timer wheel APIs.
>> Right now we use add_timer or add_timer_on, to add a timer to any
>> or to a specific CPU respectively.
>> Would it be useful to have an interface like try_add_timer_on, that
>> would return an error or would use add_timer, if the specified CPU is
>> offline.
>>
>> Recently we have come across some bugs in the RDS code, where a delayed
>> work was being queued on an offlined CPU and as a result of that the
>> underlying timer was not firing, which in turn meant that the work was
>> never able to make it to the intended worker_pool.
> 
> Urgh. Clearly add_timer_on() lacks a check and a warning for that.
> 
>> I understand that this is something that needs fixing at caller side and
>> we are taking that approach.
>>
>> But I also wanted to understand if there is some scope of change on timer
>> side, for such situations. I saw your reply in [1] and agree with your point.
>> But that conversation is more than a decade old, so I thought of asking this
>> question, assuming that there may be some other use cases that can utilize
>> this new interface.
>>
>> One can also ask to change queue_delayed_work_on or have an equivalent,
>> that would check if CPU is online before doing add_timer_on but I am not sure
>> if workqueue is the only subsystem that can run into this situation.
> 
> I have no idea.
> 
> That said, I'm not opposed to have a function like
> timer_try_add_on_cpu(), which validates that the CPU is online and
> returns true if the timer is queued or false if the target CPU is
> offline. Then the call site can decided what to do with that
> situation. That's way better than queueing it on some randomly picked
> online CPU silently.
> 

I have sent a couple of patches at [1] in this regard. Could you kindly
have a look and let me know your feedback.

Thanks,
Imran

[1]: https://lore.kernel.org/all/20250115134111.2703089-1-imran.f.khan@oracle.com/