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Scheduling in Android

This document provides an overview of scheduling in Android. It discusses the architecture of Android and how it interacts with the Linux scheduler. It covers the history of Linux scheduling including the Completely Fair Scheduler (CFS) and scheduling classes. It also discusses how Android has had problems with the Linux scheduler responding quickly enough to changes, and the solutions developed to address this including new governors and control groups. Finally, it summarizes how the Android framework interacts with the Linux scheduler through system services and process scheduling policies.

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1 Scheduling in Android AnDevCon San Francisco 2016 Karim Yaghmour @karimyaghmour karim.yaghmour@opersys.com BETA
2 These slides are made available to you under a Creative Commons Share- Alike 3.0 license. The full terms of this license are here: https://creativecommons.org/licenses/by-sa/3.0/ Attribution requirements and misc., PLEASE READ: ● This slide must remain as-is in this specific location (slide #2), everything else you are free to change; including the logo :-) ● Use of figures in other documents must feature the below “Originals at” URL immediately under that figure and the below copyright notice where appropriate. ● You are free to fill in the “Delivered and/or customized by” space on the right as you see fit. ● You are FORBIDEN from using the default “About” slide as-is or any of its contents. ● You are FORBIDEN from using any content provided by 3rd parties without the EXPLICIT consent from those parties. (C) Copyright 2016, Opersys inc. These slides created by: Karim Yaghmour Originals at: www.opersys.com/community/docs Delivered and/or customized by
3 About ● Author of: ● Introduced Linux Trace Toolkit in 1999 ● Originated Adeos and relayfs (kernel/relay.c) ● Training, Custom Dev, Consulting, ...
4 Agenda 1. Architecture 2. Linux scheduler history 3. Completely Fair Scheduler (CFS) 4. Sched Classes 5. CPU power management 6. Load tracking 7. Android problems w/ Linux scheduler 8. User-space vs. Linux scheduler 9. Framework 10. Summing up
5 1. Architecture ● Hardware on which Android is based ● Android stack ● Startup ● System services ● Binder driver
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12 2. Linux scheduler history ● 1995 - Linux 1.2 ● Circular queue of tasks w/ round-robin ● 1999 - Linux 2.2 ● Scheduling classes: real-time, non-preemptable and non-real-time ● SMP support ● 2001 - Linux 2.4 O(N) scheduler ● Each task gets a slice (epoch) ● 2003 - Linux 2.6 O(1) scheduler ● Two queues: active vs. Expired ● Heuristics-based / no “formal” algorithm ● 2007 - ~ Linux 2.6.21 ● Con Kolivas' Rotating Staircase Deadline Scheduler (RSDL) ● 2007 - Linux 2.6.23 ● Ingo Molnar's Completely Fair Scheduler (CFS)
13 3. Completely Fair Scheduler (CFS) ● Tasks put in red-black tree ● Self-balancing – i.e. no path more than twice other path ● O (log n) operations ● task_struct->sched_entity->rb_node ● kernel/core/sched.c:schedule() ● put_prev_task() ● pick_next_task() ● Priorities provide decay factors ● Higher priority = lower decay factor ● i.e. lower priority uses up their time more quickly ● Group scheduling – control group mechanism (cgroups) ● Grouped processes share a single process' virtual time ● Use of cgroupfs – /acct in Android
14 4. Sched Classes ● See kernel/sched/sched.h ● struct sched_class ● Classes ● CFS - SCHED_OTHER (i.e. SCHED_NORMAL in the sources) ● RT - SCHED_FIFO / SCHED_RR ● Deadline - SCHED_DEADLINE(since 3.14) ● Idle - Not!!! SCHED_IDLE ● Stop – CPU stopping ● sched_setscheduler()
15 5. CPU power management ● Need to integrate CPU power management and scheduler more closely ● Existing Linux support for power management (governors) ● cpuidle – What happens when the CPU is IDLe ● cpufreq drivers – HW counters- / FW-based decision (intel_pstate / longrun) – Hard-coded values based on ranges within driver DVFS – “Dynamic Voltage and Frequency Scaling” ● Trivial – Performance - highest frequency – Powersave - lowest frequency – Userspace - User-space app makes decision ● Non-trivial – Based on system load ● Stats for non-trivial governors come from scheduler stats (/proc/stat) ● On-demand - Immediate jump on load increase ● Conservative - Gradual scale-up on increase
16 5.1. Making frequency scaling choices ● SCHED_DEADLINE ● We have precise info, because of how this works ● SCHED_FIFO / SCHED_RR ● Put CPU at max right away ● SCHED_NORMAL ● Use stats
17 6. Load tracking ● How to track how much “load” a process is putting on the system? ● Not just CPU time consumed ● Process waiting for CPU is contributing to load ● Since 3.8 ● PELT - Per-Entity Load Tracking – Each load instance is .y times the previous one: ● L0 + L1*y + L2*y2 + L3*y3 + ... ● New proposal (used in Pixel) ● WALT - Window-Assisted Load Tracking
18 7. Android problems w/ Linux scheduler ● Even “on-demand” isn't good enough ● It takes too many milliseconds for timer to tick and stats to be updated ● Since decision is based on stats, delay is user- noticeable
19 7.1. Android Initial Solution ● Android devs wrote their own governor: ● “interactive” ● Detects transition out of idle ● Shortens timeout to scale up to much shorter time (“1-2 ticks”) ● Out of tree
20 7.2. Intermediate solutions discussed ● Trigger cpufreq when scheduler updates stats ● Instead of trigger using a timeout ● Introduce new governor: schedutil (merged in 4.7) ● Use the info straight from the scheduler stats update
21 7.3. Recent Work ● Problems: ● bigLITTLE ● EAS – Energy Aware Schedulign ● SchedTune Used Pixel phone ● Provide user-space knobs to control schedutil governor ● Android doesn't need to make suppositions, it knows what's happening ● Implemented as control-group controller ● Each cgroup has tunable “schedtune.boost” ● sched-freq ● Use CFS runqueue info to scale CPU freq ● WALT – Window-Assisted Load Tracking
22 8. User-space vs. Linux scheduler ● Control knobs ● /dev/cpuset/ - which tasks run on which CPUs ● /dev/cpuctl/ - restrict CPU time for bg tasks ● /dev/stune/ - EAS stune boosting ● init.rc ● Power HAL: ● Power “hints” ● Set interactive
23 9. Framework ● System Services: ● Activity Manager – Causes apps to be started through Zygote – Feed lifecycle events – Manages foreground vs. background, etc. ● Scheduling Policy: – Modifies process scheduler based on request ● Defined thread groups: ● Default ● BG non-interactive ● Foreground ● System ● Audio App -- SCHED_FIFO ● Audio Sys -- SCHED_FIFO ● Top App ● See ● frameworks/base/core/java/android/os/Process.java ● frameworks/base/core/jni/android_util_Process.cpp
24 10. Summing Up ● Quite a few moving parts ● Linux does bulk of work ● Android gives hints to Linux ● Still ongoing work to get best performance with least battery usage
25 Thank you ... karim.yaghmour@opersys.com