On Wed, Oct 28, 2026 at 04:58:47PM +0100, Frederic Weisbecker wrote: > Le Sat, Oct 24, 2026 at 12:32:38AM +0800, Peng Wang a écrit : > > On Fri, Oct 23, 2026 at 05:00:22PM +0200, Frederic Weisbecker wrote: > > > Le Sat, Oct 17, 2026 at 09:13:41PM +0800, Peng Wang a 殂rit : > > > > Cloud hosts may pin vCPU threads one-to-one to host CPUs. At full > > > > allocation this includes the housekeeping CPUs, which always retain > > > > their non-idle tick. Operators want comparable performance for otherwise > > > > equivalent vCPUs. Retaining the non-idle tick on nohz_full CPUs as well > > > > avoids one placement-dependent difference in host tick interference. > > > > With spare capacity, selected CPUs may stop the tick to reduce host > > > > scheduler-tick interference with vCPUs. > > > > > > > > Allocation changes as VMs are created or destroyed while other VMs > > > > remain running. A vCPU thread may be the only runnable task in either > > > > state, so existing tick dependencies do not express this policy. > > > > > > I'm not sure I understand correctly what you are trying to solve. > > > Are you saying that you want to keep the tick running when there > > > is more than one vCPU per CPU? Because the scheduler already takes > > > care of that. > > > > Hi Frederic, > > > > No. We pin vCPU threads one-to-one to host CPUs, without > > oversubscription. By "full allocation" I meant that all allocatable > > host CPUs are in use, including CPUs outside the nohz_full set. > > > > For example, on an eight-CPU host booted with isolcpus=nohz,1-7, > > CPU0 is outside the nohz_full set: > > > > - With eight pinned vCPUs, CPU0 retains its non-idle tick while > > CPUs1-7 may stop theirs. Retaining the tick on CPUs1-7 too avoids > > this placement-dependent difference in tick interference between > > otherwise equivalent vCPUs. > > > > - With seven or fewer pinned vCPUs, we can leave CPU0 without a > > vCPU and keep them on CPUs1-7. We can then allow tick suppression > > on CPUs1-7, subject to the existing dependencies. This gives > > these vCPUs a common tick policy while reducing host tick > > interference. > > > > If the vCPU on CPU0 is released while the other seven remain > > running, we allow CPUs1-7 to suppress their ticks. If a new vCPU > > is later assigned to CPU0, we retain the ticks on CPUs1-7 again > > to avoid the difference in tick interference. > > > > Throughout these transitions, CPU1 can still have its original > > pinned vCPU thread as its only runnable task. Its runnable task > > count does not change merely because CPU0 gains or loses a vCPU, > > so the scheduler's existing tick dependency cannot express this > > allocation policy. > > Ok now I understand what you want to do, I just don't understand why. > Why it matters to turn off nohz_full on all CPUs when vCPUs run on > housekeeping? > > > > > Add a per-CPU POLICY dependency within the boot-time nohz_full set. > > > > Permission defaults to enabled. The policy can be changed without > > > > taking CPUs offline. Revoking permission uses the existing asynchronous > > > > kick; granting it leaves all other dependencies effective. Preserve > > > > POLICY across CPU hotplug and report it in tick_stop tracing. > > > > > > > > Idle tick suppression, nohz_full membership, context tracking, RCU > > > > offloading and housekeeping configuration are unchanged. > > > > > > This still keeps the overhead of nohz_full even if the tick stays on, > > > right? > > > (RCU nocb, kernel <-> user overhead, etc...). Isn't it a problem for you? > > > > Yes. Our setup uses only the nohz flag of isolcpus, without > > additional housekeeping affinity tuning. Context tracking and > > RCU callback offloading remain enabled when the tick is kept > > running. > > > > We have observed a performance benefit from this nohz_full setup > > with our database workload running on pinned vCPUs. >
Hi Frederic, > Do you mean that your database runs better when hosts CPUs are in nohz_full? Yes. In our database tests, suppressing the host's non-idle periodic tick improved performance, with the rest of the host configuration unchanged. > But how does this new interface help? Ie: why is it worse to have all CPUs in The tick policy itself can therefore affect guest performance. A large host runs VMs for many customers with different workloads. Customers create and release VMs independently, so the same host CPU can be reassigned to a different workload while other VMs remain running. We want to choose whether to retain the non-idle tick or allow its suppression for the workload currently assigned to each CPU, and revisit that choice when the CPU is reassigned. Per-CPU control lets us do this without rebooting or taking CPUs offline. > nohz_full except the housekeeping rather than shutting down nohz_full > altogether? > > Or to put it another way: why is it better to have either: > > - all vCPUs on nohz_full host CPUs > or > - all vCPUs on ticking CPUs > > And why does it hurt to have a single vCPU running on a ticking CPU. > If a VM's vCPUs span housekeeping and nohz_full CPUs, some may benefit from tick suppression while others cannot. For workloads sensitive to tick interference, this can cause performance differences within the VM. A common tick policy is one deployment option in that case. In summary, the runtime control would let us: 1. Select or withdraw tick suppression as CPUs are assigned to different workloads. 2. Apply a common non-idle tick policy across a VM's vCPUs when preferred. Thanks, Peng > Thanks. > > -- > Frederic Weisbecker > SUSE Labs
