[PATCH v3 0/5] s390/idle: CPU idle driver

[Mete Durlu <meted@linux.ibm.com>]

v2 -> v3:
* Link to v2: https://lore.kernel.org/r/20260609-idledriver-v2-0-21c2f581d11f@linux.ibm.com
* Remove any mentions of idle governor since the driver is not tied to
  any specific governor. TEO is just the default pick for s390
  (Christian Loehle)
* Add a comment to reflect how the values for idle driver tuneables are
  picked (Christian Loehle)

v1 -> v2:

* Add idle driver enteries to MAINTAINERS file (Christian Loehle)
* Remove extra line break left in drivers/cpuidle/Kconfig.s390
  (Christian Loehle)

This patch series introduces a CPU idle driver for s390 architecture
that leverages the existing cpu idle infrastructure and idle governor
to optimize idle state selection based on timer events and interrupt
patterns.

- Implementation Overview
-----------------------------------------------------------------------

The driver implements two idle states;

**Polling Idle (shallow state)**
- Busy-loop implementation that prevents SIE-exit during short idle periods
- Zero exit latency, optimized for latency-sensitive workloads
- Maintains CPU with the guest, improving response times

**Enabled Wait (deep state)**
- Traditional EW state that signals hypervisor availability
- Higher exit latency but more efficient for longer idle periods

The idle governor dynamically selects between these states by monitoring
next timer interrupts and system interrupt frequency, choosing the most
appropriate idle state for each situation.

- Performance Benefits
-----------------------------------------------------------------------

Benchmark results across LPAR, KVM, and z/VM guests demonstrate significant
improvements for latency-sensitive and scheduler-intensive workloads:

```
Benchmark    | LPAR  | z/VM  | KVM   |
-----------------------------------------
cyclictest   | +45%  | +50%  | +30%  |
hackbench    | +5%   | +12%  | -     |
pgbench      | +2%   | +15%  | +2%   |
uperf        | +50%* | +-5%  | +-5%  |
```

*Single-threaded uperf scenarios show 50-70% improvements on LPAR.
Multi-threaded results show slight improvements overall.

Additionally, fio and ioping tests show latency distribution shifts toward
lower values with reduced minimum and average latencies.

- Configuration
-----------------------------------------------------------------------

Target residency and exit latency values are benchmark estimates. Due to
s390's virtualized architecture, these values are non-deterministic.
Idle state parameters are tuned per hypervisor type after benchmarks:

**LPAR:**
- EW: 5us target residency, 5us exit latency

**KVM/z/VM:**
- EW: 1us target residency, 1us exit latency

---
Mete Durlu (5):
      s390/tick: Remove CIF_NOHZ_DELAY flag
      tick: Remove arch_needs_cpu
      s390: Enable TIF_POLLING_NRFLAG
      s390/idle: Introduce cpuidle for s390
      s390/configs: Enable cpuidle driver on s390

 MAINTAINERS                         |   8 +++
 arch/s390/Kconfig                   |   5 ++
 arch/s390/configs/debug_defconfig   |   2 +
 arch/s390/configs/defconfig         |   2 +
 arch/s390/include/asm/processor.h   |   4 --
 arch/s390/include/asm/thread_info.h |   1 +
 arch/s390/kernel/idle.c             |   1 -
 arch/s390/kernel/irq.c              |   4 --
 arch/s390/kernel/smp.c              |   1 -
 drivers/cpuidle/Kconfig             |   5 ++
 drivers/cpuidle/Kconfig.s390        |  11 ++++
 drivers/cpuidle/Makefile            |   4 ++
 drivers/cpuidle/cpuidle-s390.c      | 115 ++++++++++++++++++++++++++++++++++++
 include/linux/tick.h                |   3 -
 kernel/time/tick-sched.c            |   7 +--
 15 files changed, 156 insertions(+), 17 deletions(-)
---
base-commit: 2d3090a8aeb596a26935db0955d46c9a5db5c6ce
change-id: 20260609-idledriver-217e2b2712a3

Best regards,
-- 
Mete Durlu <meted@linux.ibm.com>