Re: [PATCH] workqueue: warn if delayed_work is queued to an offlined cpu.

[imran.f.khan@oracle.com]

Hello Tejun,
Thanks a lot for having a look.
On 10/1/2025 8:08 am, Tejun Heo wrote:
> On Thu, Jan 09, 2025 at 09:18:29PM +1100, Imran Khan wrote:
>> delayed_work submitted to an offlined cpu, will not get executed,
>> after the specified delay if the cpu remains offline. If the cpu
>> never comes online the work will never get executed.
>> checking for online cpu in __queue_delayed_work, does not sound
>> like a good idea because to do this reliably we need hotplug lock
>> and since work may be submitted from atomic contexts, we would
>> have to use cpus_read_trylock. But if trylock fails we would queue
>> the work on any cpu and this may not be optimal because our intended
>> cpu might still be online.
>>
>> Putting a WARN_ON for an already offlined cpu, will indicate users
>> of queue_delayed_work_on, if they are (wrongly) trying to queue
>> delayed_work on offlined cpu.
>>
>> Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
>> ---
>>  kernel/workqueue.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
>> index 8e0bb3c608239..10878b5e3d74f 100644
>> --- a/kernel/workqueue.c
>> +++ b/kernel/workqueue.c
>> @@ -2508,6 +2508,7 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
>>  		return;
>>  	}
>>  
>> +	WARN_ON(cpu != WORK_CPU_UNBOUND && !cpu_online(cpu));
> 
> Can we use WARN_ON_ONCE() instead?
> 
Yes we can. The reason I was using WARN_ON was that, if 
someone does this for multiple offlined CPU, we could see the
warnings for each of them.

I have sent a v2 of this patch with WARN_ON_ONCE. Also in v2, 
I have modified description of queue_delayed_work_on, similar to
that of queue_work_on, to indicate why the caller should ensure
that specified cpu is and remains online.

Thanks,
Imran

> Thanks.
>