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McKenney wrote: > On Tue, Jan 13, 2026 at 08:57:20AM +0000, Joel Fernandes wrote: >> >> >>> On Jan 12, 2026, at 11:55 PM, Shrikanth Hegde wrote: >>> >>> Hi. >>> >>> On 1/13/26 8:16 AM, Joel Fernandes wrote: >>> >>> >>>>>>>> Another way to make it in-kernel would be to make the RCU normal wake >>>>>>>> from GP optimization enabled for > 16 CPUs by default.>> >>>>>>>> I was considering this, but I did not bring it up because I did not >>>>>>>> know that there are large systems that might benefit from it until now.> >>>>>>> This would require increasing the scalability of this optimization, >>>>>>> right? Or am I thinking of the wrong optimization? ;-) >>>>>>> >>>>>> Yes I think you are considering the correct one, the concern you have is >>>>>> regarding large number of wake ups initiated from the GP thread, correct? >>>>>> >>>>>> I was suggesting on the thread, a more dynamic approach where using >>>>>> synchronize_rcu_normal() until it gets overloaded with requests. One approach >>>>>> might be to measure the length of the rcu_state.srs_next to detect an overload >>>>>> condition, similar to qhimark? Or perhaps qhimark itself can be used. And under >>>>>> lightly loaded conditions, default to synchronize_rcu_normal() without checking >>>>>> for the 16 CPU count. >>>>>> >>>>>> Thoughts? >>>>> >>>>> Or maintain multiple lists. Systems with 1000+ CPUs can be a bit >>>>> unforgiving of pretty much any form of contention. >>>> Makes sense. We could also just have a single list but a much smaller threshold for switching synchronize_rcu_normal off. >>>> That would address the conveyor belt pattern Vishal expressed. >>>> thanks, >>>> - Joel >>> >>> Wouldn't that make most of the sync_rcu calls on large system >>> with synchronize_rcu_normal off? >> >> It would and that is expected. >> >>> >>> Whats the cost of doing this? >> >> There is no cost, that is the point right. The scalability issue Paul is referring to is the >> large number of wake ups. You wont have that if the number of synchronous callers is small. > > Also the contention involved in the list management, if there is still > only the one list. > Even if the number of synchronize_rcu() in flight is a small number? like < 10. To clarify, I meant keeping the threshold that small in favor of the list contention issue you're raising. Thanks! - Joel