Re: [RFC PATCH 0/2] tick/nohz: Add runtime control of non-idle tick suppression

[Peng Wang <peng_wang@linux.alibaba.com>]

On Thu, Oct 15, 2026 at 12:00:25PM -0400, Waiman Long wrote:
> On 10/15/26 8:43 AM, Peng Wang wrote:
> > Workloads and their need for periodic ticks can change after boot.
> > This series lets userspace choose at runtime which CPUs in the boot-time
> > nohz_full set retain the non-idle scheduler tick and which are permitted
> > to stop it, while preserving the existing isolation configuration.
> > 
> > The first patch adds a per-CPU tick dependency; the second exposes a
> > sysfs CPU list. Permission defaults to all nohz_full CPUs. Revocation
> > uses the existing asynchronous kick. Idle tick suppression is unchanged,
> > and policy persists across hotplug.
> So you are basically setting up more nohz_full CPUs than the system actually
> needs to use at the moment and so want to disable nohz/tick part of
> nohz_full CPUs that don't need it. Can you elaborate more about the problems
> solve by introducing this new behavior?

Hi Waiman,

Yes. We have CPU-pinned workloads that benefit from reduced periodic
tick interference. The desired tick policy changes with CPU allocation.

With spare capacity, we allow selected nohz_full CPUs to stop their
tick. When capacity is fully allocated, we prefer to retain the
non-idle periodic tick across CPUs, avoiding an additional difference
in tick interference between housekeeping and nohz_full CPUs.
A CPU may still have only one runnable task in either case, so the
existing tick dependencies do not express this policy.

This series keeps the boot-time nohz_full CPU set, context tracking,
RCU offloading and housekeeping configuration unchanged. It only
controls whether those CPUs may stop their non-idle tick. The policy
can be changed without taking CPUs offline.

> > 
> > Existing cpuset partition modes do not express this independent tick
> > policy. Feedback on the proposed interface is welcome.
> 
> We are planning to eventually extend cpuset partition mode to change
> nohz_full at runtime. This is still a work in progress.

Thanks for mentioning the cpuset work. Would this tick-only policy
fit into that plan? The core mechanism in patch 1 is independent of
the sysfs interface.

Thanks,
Peng

> 
> Regards,
> Longman
> 
> > 
> > Thanks,
> > Peng
> > 
> > Peng Wang (2):
> >    tick/nohz: Add a policy dependency for full dynticks
> >    cpu: Add sysfs control of full dynticks policy
> > 
> >   .../ABI/testing/sysfs-devices-system-cpu      | 40 ++++++++++++
> >   drivers/base/cpu.c                            | 46 ++++++++++++++
> >   include/linux/tick.h                          |  8 ++-
> >   include/trace/events/timer.h                  |  3 +-
> >   kernel/time/tick-sched.c                      | 62 +++++++++++++++++++
> >   5 files changed, 156 insertions(+), 3 deletions(-)
> > 
> > 
> > base-commit: 0fcb05b72344f40806cbe810de8872f1698bf053
>