> On Jan 13, 2026, at 7:19 AM, Uladzislau Rezki <[email protected]> wrote: > > On Mon, Jan 12, 2026 at 05:36:24PM +0000, Joel Fernandes wrote: >> >> >>>> On Jan 12, 2026, at 12:09 PM, Uladzislau Rezki <[email protected]> wrote: >>> >>> On Mon, Jan 12, 2026 at 04:09:49PM +0000, Joel Fernandes wrote: >>>> >>>> >>>>>> On Jan 12, 2026, at 7:57 AM, Uladzislau Rezki <[email protected]> wrote: >>>>> >>>>> Hello, Shrikanth! >>>>> >>>>>> >>>>>>> On 1/12/26 3:38 PM, Uladzislau Rezki wrote: >>>>>>> On Mon, Jan 12, 2026 at 03:13:33PM +0530, Vishal Chourasia wrote: >>>>>>>> Bulk CPU hotplug operations—such as switching SMT modes across all >>>>>>>> cores—require hotplugging multiple CPUs in rapid succession. On large >>>>>>>> systems, this process takes significant time, increasing as the number >>>>>>>> of CPUs grows, leading to substantial delays on high-core-count >>>>>>>> machines. Analysis [1] reveals that the majority of this time is spent >>>>>>>> waiting for synchronize_rcu(). >>>>>>>> >>>>>>>> Expedite synchronize_rcu() during the hotplug path to accelerate the >>>>>>>> operation. Since CPU hotplug is a user-initiated administrative task, >>>>>>>> it should complete as quickly as possible. >>>>>>>> >>>>>>>> Performance data on a PPC64 system with 400 CPUs: >>>>>>>> >>>>>>>> + ppc64_cpu --smt=1 (SMT8 to SMT1) >>>>>>>> Before: real 1m14.792s >>>>>>>> After: real 0m03.205s # ~23x improvement >>>>>>>> >>>>>>>> + ppc64_cpu --smt=8 (SMT1 to SMT8) >>>>>>>> Before: real 2m27.695s >>>>>>>> After: real 0m02.510s # ~58x improvement >>>>>>>> >>>>>>>> Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b >>>>>>>> >>>>>>>> [1] >>>>>>>> https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.ca...@linux.vnet.ibm.com >>>>>>>> >>>>>>> Also you can try: echo 1 > >>>>>>> /sys/module/rcutree/parameters/rcu_normal_wake_from_gp >>>>>>> to speedup regular synchronize_rcu() call. But i am not saying that it >>>>>>> would beat >>>>>>> your "expedited switch" improvement. >>>>>>> >>>>>> >>>>>> Hi Uladzislau. >>>>>> >>>>>> Had a discussion on this at LPC, having in kernel solution is likely >>>>>> better than having it in userspace. >>>>>> >>>>>> - Having it in kernel would make it work across all archs. Why should >>>>>> any user wait when one initiates the hotplug. >>>>>> >>>>>> - userspace tools are spread across such as chcpu, ppc64_cpu etc. >>>>>> though internally most do "0/1 > /sys/devices/system/cpu/cpuN/online". >>>>>> We will have to repeat the same in each tool. >>>>>> >>>>>> - There is already /sys/kernel/rcu_expedited which is better if at all >>>>>> we need to fallback to userspace. >>>>>> >>>>> Sounds good to me. I agree it is better to bypass parameters. >>>> >>>> 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. >>>> >>> IMO, we can increase that threshold. 512/1024 is not a problem at all. >>> But as Paul mentioned, we should consider scalability enhancement. From >>> the other hand it is also probably worth to get into the state when we >>> really see them :) >> >> Instead of pegging to number of CPUs, perhaps the optimization should be >> dynamic? That is, default to it unless synchronize_rcu load is high, default >> to the sr_normal wake-up optimization. Of course carefully considering all >> corner cases, adequate testing and all that ;-) >> > Honestly i do not see use cases when we are not up to speed to process > all callbacks in time keeping in mind that it is blocking context call. > > How many of them should be in flight(blocked contexts) to make it starve... :) > According to my last evaluation it was ~64K. > > Note i do not say that it should not be scaled.
But you did not test that on large system with 1000s of CPUs right? So the options I see are: either default to always using the optimization, not just for less than 17 CPUs (what you are saying above). Or, do what I said above (safer for system with 1000s of CPUs and less risky). Let me know if I missed something. Thanks. > > -- > Uladzislau Rezki
