Kernel Recipes 2018 - CPU Idle Loop Rework - Rafael J. Wysocki
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The document discusses the rework of the CPU idle loop, presented by Rafael J. Wysocki at Kernel Recipes 2018, which addresses issues in managing CPU idle states for power efficiency. It highlights the original design problems, particularly in predicting idle durations and the complications introduced by the CPU scheduler. The revised solution implemented in Linux kernel 4.17 improves idle power savings and overall system performance.
![Introduction CPU Idle States
CPU Idle States Properties
Time [a. u.]
Power [a. u.]
Entry ExitMR
Residency
Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 4 / 19](https://image.slidesharecdn.com/cpuidlelooprework-181002203428/85/Kernel-Recipes-2018-CPU-Idle-Loop-Rework-Rafael-J-Wysocki-4-320.jpg)
Kernel Recipes 2018 - CPU Idle Loop Rework - Rafael J. Wysocki
- 1. CPU Idle Loop Rework Rafael J. Wysocki Intel Open Source Technology Center September 26, 2018 Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 1 / 19
- 2. Introduction CPU Idle Time Concept Terminology: CPUs = Logical CPUs CPU 0 CPU 1 CPU 2 CPU 3 CPU 0 CPU 1 CPU 0 Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 2 / 19
- 3. Introduction CPU Idle Time Concept CPUs: Busy Vs Idle Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 3 / 19
- 4. Introduction CPU Idle States CPU Idle States Properties Time [a. u.] Power [a. u.] Entry ExitMR Residency Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 4 / 19
- 5. Introduction CPU Idle States CPU Idle States Example: C-states On Intel Hardware MWAIT <HINT> CPU 0 CPU 1 CPU 2 CPU 3 CPU 4 CPU 5 CPU 6 CPU 7 C6C2 C3 C3 C6C6 C6 C6 CC2 CC3 CC6 CC6 PC2 Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 5 / 19
- 6. CPU Idle Time Management Original Design and Problem High-level CPU Idle Time Management Control Flow CPU scheduler Select state Estimate idle time CPU is idle Enter state Governor YES NO menu ladder ACPI idle intel_idle Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 6 / 19
- 7. CPU Idle Time Management Original Design and Problem Complication: CPU Scheduler Tick Timer Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 7 / 19
- 8. CPU Idle Time Management Original Design and Problem Original Idle Loop Design Issue CPU scheduler Try to stop tick need_resched()? YES NO Select state Estimate idle time Governor Restart tick Enter state Interrupt May be too short! Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 8 / 19
- 9. CPU Idle Time Management Original Design and Problem Short Idle Duration Prediction Problem WIN LOSS LOSS LOSS Short Idle Short Idle Long Idle Long Idle Predicted Actual Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 9 / 19
- 10. CPU Idle Time Management Original Design and Problem Idle Governor Problem: Two Categories Of Information Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 10 / 19
- 11. CPU Idle Time Management Original Design and Problem The menu Idle Governor Next Timer Event CPU scheduler Utilization Information NOHZ Infrastructure Idle Time Range Latency LimitPredicted Idle Duration Idle State Previous Idle Duration Wakeup Statistics Timers Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 11 / 19
- 12. CPU Idle Time Management Idle Loop Rework Redesigned Idle Loop (Linux* 4.17 And Later) CPU scheduler Try to stop tick need_resched()? NO Select state Estimate idle time Governor Restart tick Enter state Interrupt Stop tick or tick stopped? YES NO Tick stopped? NO YES * Other names and brands may be claimed as the property of others Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 12 / 19
- 13. CPU Idle Time Management Idle Loop Rework Short Idle Duration Prediction Problem Mitigated WIN LOSS Neutral WIN Short Idle Short Idle Long Idle Long Idle Predicted Actual Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 13 / 19
- 14. CPU Idle Time Management Idle Loop Rework Idle Power (Intel OTC Server Power Lab) Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 14 / 19
- 15. CPU Idle Time Management Idle Loop Rework Idle Power (Intel OTC Server Power Lab) Continued Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 15 / 19
- 16. CPU Idle Time Management Idle Loop Rework Idle Power (Intel OTC Server Power Lab) Continued Before After (different scale) Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 16 / 19
- 17. Questions? Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 17 / 19
- 18. Resources References Linux Kernel Bug 199227, Idle loop ordering issue (https://bugzilla.kernel.org/show_bug.cgi?id=199227). T. Ilsche, M. H¨ahnel, R. Sch¨ohne, M. Bielert, D. Hackenberg, Powernightmares: The Challenge of Efficiently Using Sleep States on Multi-Core Systems (https://tu-dresden.de/zih/forschung/ressourcen/dateien/projekte/haec/powernightmares.pdf?lang=en). R. J. Wysocki, Power Management Challenges in Linux (https: //www.linuxplumbersconf.org/2017/ocw//system/presentations/4652/original/linux_pm_challenges.pdf). R. J. Wysocki, PM Infrastructure in the Linux Kernel – Current Status and Future (https://events.linuxfoundation.org/sites/events/files/slides/kernel_PM_infra_0.pdf). J. Corbet, The cpuidle subsystem (http://lwn.net/Articles/384146/). Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 18 / 19
- 19. Disclaimer Intel technologies’ features and benefits depend on system configuration and may require enabled hardware, software or service activation. Performance varies depending on system configuration. No computer system can be absolutely secure. Check with your system manufacturer or retailer or learn more at www.intel.com. Intel and the Intel logo are trademarks of Intel Corporation in the U.S. and/or other countries. *Other names and brands may be claimed as the property of others. c Intel Corporation Rafael J. Wysocki CPU Idle @ Kernel Recipes 2018 September 26, 2018 19 / 19