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Message-ID: <aWaH1AdJeSVVB5UZ@milan> Date: Tue, 13 Jan 2026 18:58:44 +0100 From: Uladzislau Rezki <urezki@...il.com> To: Joel Fernandes <joelagnelf@...dia.com> Cc: Uladzislau Rezki <urezki@...il.com>, Shrikanth Hegde <sshegde@...ux.ibm.com>, Vishal Chourasia <vishalc@...ux.ibm.com>, "rcu@...r.kernel.org" <rcu@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "paulmck@...nel.org" <paulmck@...nel.org>, "frederic@...nel.org" <frederic@...nel.org>, "neeraj.upadhyay@...nel.org" <neeraj.upadhyay@...nel.org>, "josh@...htriplett.org" <josh@...htriplett.org>, "boqun.feng@...il.com" <boqun.feng@...il.com>, "rostedt@...dmis.org" <rostedt@...dmis.org>, "tglx@...utronix.de" <tglx@...utronix.de>, "peterz@...radead.org" <peterz@...radead.org>, "srikar@...ux.ibm.com" <srikar@...ux.ibm.com> Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug operations On Tue, Jan 13, 2026 at 09:32:13AM -0500, Joel Fernandes wrote: > > > On 1/13/2026 9:17 AM, Uladzislau Rezki wrote: > > On Tue, Jan 13, 2026 at 12:44:10PM +0000, Joel Fernandes wrote: > >> > >> > >>> On Jan 13, 2026, at 7:19 AM, Uladzislau Rezki <urezki@...il.com> wrote: > >>> > >>> On Mon, Jan 12, 2026 at 05:36:24PM +0000, Joel Fernandes wrote: > >>>> > >>>> > >>>>>> On Jan 12, 2026, at 12:09 PM, Uladzislau Rezki <urezki@...il.com> wrote: > >>>>> > >>>>> On Mon, Jan 12, 2026 at 04:09:49PM +0000, Joel Fernandes wrote: > >>>>>> > >>>>>> > >>>>>>>> On Jan 12, 2026, at 7:57 AM, Uladzislau Rezki <urezki@...il.com> 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.camel@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? > >> > > No, no. I do not have access to such systems. > > > >> > >> 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). > >> > > You mean introduce threshold and count how many nodes are in queue? > > Yes. > > > To me it sounds not optimal and looks like a temporary solution. > > Not more sub-optimal than the existing 16 CPU hard-coded solution I suppose. > It was trial testing :) Agree we should do something with it. > > > > Long term wise, it is better to split it, i mean to scale. > > But the scalable solution is already there: the !synchronize_rcu_normal path, > right? And splitting the list won't help this use case anyway. > Fair point. > > > > Do you know who can test it on ~1000 CPUs system? So we have some figures. > > I don't have such systems either. The most I can go is ~200+ CPUs. Perhaps the > folks on this thread have such systems as they mentioned 1900+ CPU systems. They > should be happy to test. > > > > > What i have is 256 CPUs system i can test on. > Same boat. ;-) > :) -- Uladzislau Rezki
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