Writing a Linux scheduler in Java with eBPF

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What is scheduling?

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Process A Process B CPU 1 ... CPU 2 ...

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Process A Process B CPU Time

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Process A Process B CPU A Time

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Process A Process B CPU A Time

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Process A Process B CPU A Time B

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Process A Process B CPU A Time B A

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Process A Process B CPU A Time B A A

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Process A Process B CPU A Time B A B A

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Process A Process B CPU A Time B A B A A

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Runs on this machine

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What is the performance? Good* * For a typical Java benchmark

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Why?

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1.Ease of experimentation and exploration 2.Customization 3.Rapid scheduler deployments https://lwn.net/Articles/978911/ “

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How does it work?

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CPU sched-ext User land Kernel land Hardware Java Code C Code Byte Code Loaded attach

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Show me the code

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@BPF(license = "GPL") public abstract class SampleScheduler extends BPFProgram implements Scheduler, Runnable { static final long SHARED_DSQ_ID = 0; @Type static class Stats { long global; long local; } @Override public int init() { return scx_bpf_create_dsq(SHARED_DSQ_ID, -1); } }

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@BPF(license = "GPL") public abstract class SampleScheduler { @BPFMapDefinition(maxEntries = 100) BPFHashMap statsPerCPU; @BPFMapDefinition(maxEntries = 100000) BPFLRUHashMap<@Unsigned Integer, @Unsigned Long> enqueuesPerProcess; }

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@BPF(license = "GPL") public abstract class SampleScheduler { @Override public int selectCPU(Ptr p, int prev_cpu, long wake_flags) { boolean is_idle = false; int cpu = scx_bpf_select_cpu_dfl(p, prev_cpu, wake_flags, Ptr.of(is_idle)); if (is_idle) { !/* !!... / } return cpu; } } *

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@BPF(license = "GPL") public abstract class SampleScheduler { @Override public void enqueue(Ptr p, long enq_flags) { incrementStats(false); recordEnqueue(p); scx_bpf_dispatch(p, SHARED_DSQ_ID, SCX_SLICE_DFL.value(), enq_flags); } }

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@BPF(license = "GPL") public abstract class SampleScheduler { @Override public void dispatch(int cpu, Ptr prev) { scx_bpf_consume(SHARED_DSQ_ID); } }

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Why in Java?

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Why not?

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Testing

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Race conditions Process A Process B Data(uid=0, gid=0, counter=10) set get ⚡

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Deadlock Lock 1 Process A Process B own want want Lock 2 own ⚡

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+ = Better testing of concurrency

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Can we use it?

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Sched-ext is not yet in main-line

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When will sched-ext be merged? I will knock wood, because I was sure it was going to be in 6.11, but yes it looks like we're now looking at 6.12. https://www.youtube.com/watch?v=nJMEuBwMD18 https://www.facesofopensource.com/linus-torvalds/ “

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is a prototype

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Fi#. Thanks to Dylan Reimerink sched-ext Slack Johannes Bechberger mostlynerdless.de OpenJDK Developer, SAP