linux-kernel - Re: Query about timer wheel API

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Message-ID: <254db089-b5b8-4461-bd42-2cbfea7092b9@oracle.com>
Date: Fri, 27 Dec 2024 00:07:11 +1100
From: imran.f.khan@...cle.com
To: Hillf Danton <hdanton@...a.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Tejun Heo <tj@...nel.org>,
        john.stultz@...aro.org, sboyd@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: Query about timer wheel API

Hello Hillf,
On 25/12/2024 10:01 pm, Hillf Danton wrote:
> On Tue, 24 Dec 2024 23:41:48 +1100 imran.f.khan@...cle.com
>>
>> the query was not about why its (not) working with my module. The test module, in its
>> current form, is just to show that a timer-wheel timer could be inserted in timer list
>> of an offlined CPU.
> 
> Your module helps understand your query.
>>
>> What you have suggested, we are already doing it in RDS code (mentioned in my earlier messages).
>> Also just using cpu_online may not be enough, unless we do it under get/put_online_cpus.
>>
> Same pattern is in smp_call_function_single() where ckecking cpu after get_cpu().
> But different one in smp_call_on_cpu().
> 
>> If you see, my query was more towards, what should "add_timer_on" do for such cases or
>> can we  have another function like try_add_timer_on that tries to put the timer on specific
>> CPU, but puts it else where if that CPU is offline. Is it worth having such an interface 
>> or should we stick to the current approach of fixing this on the caller side.
> 
> The number of reports that timer is queued on offline cpu in 2024 alone raises 
> the (known) question -- what sense could be made by adding check of cpu in the 
> pathes like queuing work and arming timer?

Sorry, I could not understand the last part of your comment. Do you mean that its a
known issue that should be addressed or do you mean that my use case of adding delayed_work
and/or timer to offlined CPUs is an invalid use case and should be fixed on the caller side ?

Thanks,
Imran