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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/