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

[Mete Durlu]

This patch series introduces a CPU idle driver for s390
architecture that leverages the existing cpu idle infrastructure and
TEO (Timer Events Oriented) 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 TEO 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
-----------------------------------------------------------------------

Idle state parameters are tuned per hypervisor type after benchmarks:

**LPAR:**
- Polling: 5us target residency, 0us exit latency
- EW: 5us target residency, 5us exit latency

**KVM/z/VM:**
- Polling: 1us target residency, 0us exit latency  
- 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

 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        |  12 ++++
 drivers/cpuidle/Makefile            |   4 ++
 drivers/cpuidle/cpuidle-s390.c      | 104 ++++++++++++++++++++++++++++
 include/linux/tick.h                |   3 -
 kernel/time/tick-sched.c            |   7 +-
 14 files changed, 138 insertions(+), 17 deletions(-)
 create mode 100644 drivers/cpuidle/Kconfig.s390
 create mode 100644 drivers/cpuidle/cpuidle-s390.c

-- 
2.54.0