[PATCH v7 00/12] sched, steal_monitor: Introduce cpu_preferred_mask and steal-driven vCPU backoff

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Very briefly,
- Maintain set of CPUs which can be used by workload. It is denoted as
  cpu_preferred_mask
- Periodically compute the steal time. If steal time is high/low based
  on the thresholds, either reduce/increase the preferred CPUs. This is
  handled in a new driver called steal_monitor
- If a CPU is marked as non-preferred, push the task running on it if
  possible.
- Use this CPU state in wakeup and load balance to ensure tasks run
  within preferred CPUs.

For more details on idea, problem statement and performance numbers,
please refer to cover-letter of v2[2] and OSPM talk[1].

*** Please review and provide your feedback!! ***

[1]:https://youtu.be/adxUKFPlOp0
[2] v2: https://lore.kernel.org/all/20260407191950.643549-1-sshegde@linux.ibm.com/#t
[3] v6: https://lore.kernel.org/all/20260701141654.500125-1-sshegde@linux.ibm.com/#t

Thank you very much for feedback so far. This has helped the code to
evolve towards a clear abstraction layers and get simplified.
Special thanks to Yury Norov for reviewing this and improving the series
significantly. Really appreciated.

Apologies in advance if I have missed addressing any
comments. If so would be purely accidental, not in any way intentional.

base commit:
tip/sched/core at 'commit 04998aa54848 ("sched/eevdf: Delayed dequeue task can't preempt")'

v6->v7:
- Make new driver steal_monitor into 4-5 patches. (Yury Norov)
- Define CONFIG_STEAL_MONITOR and Make it select CONFIG_PREFERRED_CPU
  (Yury Norov)
- Make module parameters fixed at module load (Yury Norov)
- Make module parameters checks using set/get methods via module_param_cb - sashiko
- Simplify is_cpu_allowed. (Yury Norov)
- Added MAINTAINERS entry for new driver.
- Split nohz_full optimization into its own patch.
- Merged load balance patches.
- Use possible CPUs instead of active for steal value calculations.
- Drop __weak symbol for now. Once the need arises, framework can be
  designed at that time. (Yury Norov)
- remove whitespace in scoped_guard (Yury Norov)
- remove class check in sched_push_current_non_preferred_cpu (Yury
  Norov)
- Make empty stub to do { } while (0) in set_cpu_preferred
- Move is_migration_disabled check to sched_non_preferred_cpu_push_stop - Sashiko
- Increase the migration count only if rq changed - sashiko
- Add missing requeue work on early return - sashiko
- Use WARN_ON_ONCE for design checks instead of WARN_ON.
- Few updates to documentation, comments and changelogs.

Let me know if there is any critical information is missing
regarding new driver such as policy, documentation or missing
implementation. I have ensured checkpatch --strict is happy.

Shrikanth Hegde (12):
  sched/docs: Document cpu_preferred_mask and Preferred CPU concept
  cpumask: Introduce cpu_preferred_mask
  sysfs: Add preferred CPU file
  sched/core: Try to use a preferred CPU in is_cpu_allowed
  sched/fair: Load balance only among preferred CPUs
  sched/core: Push current task from non preferred CPU
  sched/debug: Add migration stats due to non preferred CPUs
  virt: Introduce steal monitor driver
  virt/steal_monitor: Add control knobs for handling steal values
  virt/steal_monitor: Provide functions for managing steal values
  virt/steal_monitor: Act on steal time periodically and decide on
    preferred CPUs
  sched, virt/steal_monitor: Keep tick on for faster push on nohz_full
    CPU

 .../ABI/testing/sysfs-devices-system-cpu      |  11 +
 Documentation/driver-api/index.rst            |   1 +
 Documentation/driver-api/steal-monitor.rst    | 111 ++++++++++
 Documentation/scheduler/sched-arch.rst        |  58 +++++
 MAINTAINERS                                   |   9 +
 drivers/base/cpu.c                            |   8 +
 drivers/virt/Kconfig                          |   2 +
 drivers/virt/Makefile                         |   1 +
 drivers/virt/steal_monitor/Kconfig            |  18 ++
 drivers/virt/steal_monitor/Makefile           |   6 +
 drivers/virt/steal_monitor/defaults.c         | 107 ++++++++++
 drivers/virt/steal_monitor/sm_core.c          | 202 ++++++++++++++++++
 drivers/virt/steal_monitor/sm_core.h          |  37 ++++
 include/linux/cpumask.h                       |  24 +++
 include/linux/sched.h                         |   1 +
 kernel/Kconfig.preempt                        |   3 +
 kernel/cpu.c                                  |   6 +
 kernel/sched/core.c                           | 105 ++++++++-
 kernel/sched/debug.c                          |   1 +
 kernel/sched/fair.c                           |  11 +-
 kernel/sched/sched.h                          |  20 ++
 21 files changed, 737 insertions(+), 5 deletions(-)
 create mode 100644 Documentation/driver-api/steal-monitor.rst
 create mode 100644 drivers/virt/steal_monitor/Kconfig
 create mode 100644 drivers/virt/steal_monitor/Makefile
 create mode 100644 drivers/virt/steal_monitor/defaults.c
 create mode 100644 drivers/virt/steal_monitor/sm_core.c
 create mode 100644 drivers/virt/steal_monitor/sm_core.h

-- 
2.47.3