[PATCH v6 00/23] sched: Introduce cpu_preferred_mask and steal-driven vCPU backoff

[Shrikanth Hegde]

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@xxxxxxxxxxxxx/#t
[3] v5: https://lore.kernel.org/all/20260625124648.802832-1-sshegde@xxxxxxxxxxxxx/

Thank you very much for feedback so far. This has helped the code to
evolve towards a clear abstraction layers and get simplified.
(Hopefully). 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 b2463ebf2674 ("sched/debug: Remove unused schedstats")'

v5->v6:
- Drop 1st patch. It is already in sched/core. Thanks Peter for picking
  it up.
- make cpu_preferred_mask as EXPORT_SYMBOL_GPL (Peter Zijlstra)
- Make set_preferred_cpu a NOP when CONFIG_PREFERRED_CPU=n, and still
  keep assign_cpu macro for =y case. (Peter Zijlstra and Yury Norov)
- Drop the optimization of caching the preferred state in
  select_fallback_rq. Initially had thought of splitting v5's patch into
  two. Then later found that having the cached value exposes a race
  scenario where task affinity could get reset due to cached value if mask
  changed after it was cached.
- Drop wakeup patch (Peter Zijlstra).
  No performance degradation seen.
  If CPU is non-preferred select_fallback_rq gets called in wakeup path.
  Additional checks of available_idle_cpu is not necessary. Hence the drop.
- Address CPU hotplug related concerns of accessing active_mask in 
  steal monitor driver code (sashiko)
- Address concerns over u64 overflow (sashiko)
- Make decrease_preferred_cpus work correctly if
  nohz_full=<last_set_of_CPUS. Don't assume
  housekeeping core is always at beginning, (sashiko)
- Added a optimization for common case where nohz_full=<empty>
- Fixed a few documentation nits (Randy Dunlap)
- Fixed "this patch" reference in changelogs (Peter Zijlstra)

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.

As mentioned in previous v5[3]'s cover-letter, I am looking for guidance 
on the below concern that will arise.
I think there should be a MAINTAINERS file entry for new
driver. I don't see a drivers/virt/* entry.
Either as a new entry for driver or a few file in SCHEDULER entry.
Let me know if/what I should add it.

Shrikanth Hegde (23):
  sched/docs: Document cpu_preferred_mask and Preferred CPU concept
  kconfig: Provide PREFERRED_CPU option
  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/fair: Pull the load on preferred CPU
  sched/core: Keep tick on non-preferred CPUs until tasks are out
  sched/core: Push current task from non preferred CPU
  sched/debug: Add migration stats due to non preferred CPUs
  virt/steal_monitor: Add documentation
  virt: Introduce steal monitor driver
  virt/steal_monitor: Restore to active on module disable
  virt/steal_monitor: Define steal_monitor structure
  virt/steal_monitor: Add control knobs for handling steal values
  virt/steal_monitor: Compute work at regular intervals
  virt/steal_monitor: Provide default method to get systemwide steal
    time
  virt/steal_monitor: Provide default method to inc/dec preferred CPUs
  virt/steal_monitor: Provide default method to get num of CPUs for
    steal ratio
  virt/steal_monitor: Act on steal values at regular intervals
  virt/steal_monitor: Add direction control
  virt/steal_monitor: Add design check of preferred subset of active
  virt/steal_monitor: Optimise decrease_preferred_cpus when all CPUs are
    housekeeping

 .../ABI/testing/sysfs-devices-system-cpu      |  11 ++
 Documentation/driver-api/index.rst            |   1 +
 Documentation/driver-api/steal-monitor.rst    |  99 +++++++++++++
 Documentation/scheduler/sched-arch.rst        |  56 ++++++++
 drivers/base/cpu.c                            |   8 ++
 drivers/virt/Makefile                         |   1 +
 drivers/virt/steal_monitor/Makefile           |  14 ++
 drivers/virt/steal_monitor/defaults.c         | 129 +++++++++++++++++
 drivers/virt/steal_monitor/sm_core.c          | 130 ++++++++++++++++++
 drivers/virt/steal_monitor/sm_core.h          |  33 +++++
 include/linux/cpumask.h                       |  27 +++-
 include/linux/sched.h                         |   1 +
 kernel/Kconfig.preempt                        |  14 ++
 kernel/cpu.c                                  |   6 +
 kernel/sched/core.c                           | 127 ++++++++++++++++-
 kernel/sched/debug.c                          |   1 +
 kernel/sched/fair.c                           |  12 +-
 kernel/sched/sched.h                          |  17 +++
 18 files changed, 682 insertions(+), 5 deletions(-)
 create mode 100644 Documentation/driver-api/steal-monitor.rst
 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