c20a19c946
During ttwu, the kernel may decide to skip ->select_task_rq() (e.g., when only one CPU is allowed or migration is disabled). This causes to call ops.enqueue() directly without having a chance to call ops.select_cpu(). Therefore, introduce a new flag (select_cpu_done) in the local task context to determine if ops.select_cpu() was bypassed and, in that case, attempt to find an idle CPU directly from ops.enqueue(). In the future this information will be supplied by the kernel through a special enqueue flag (SCX_ENQ_CPU_SELECTED) [1]. However, the custom flag in the local task context ensures to reliably determine the same information, even on older kernels where this flag is not available. [1] https://lore.kernel.org/lkml/20240928003840.GA2717@maniforge/T Signed-off-by: Andrea Righi <andrea.righi@linux.dev> |
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scx_bpfland
This is a single user-defined scheduler used within sched_ext, which is a Linux kernel feature which enables implementing kernel thread schedulers in BPF and dynamically loading them. Read more about sched_ext.
Overview
scx_bpfland: a vruntime-based sched_ext scheduler that prioritizes interactive workloads.
This scheduler is derived from scx_rustland, but it is fully implemented in BPF. It has a minimal user-space Rust part to process command line options, collect metrics and log out scheduling statistics. The BPF part makes all the scheduling decisions.
Tasks are categorized as either interactive or regular based on their average rate of voluntary context switches per second. Tasks that exceed a specific voluntary context switch threshold are classified as interactive. Interactive tasks are prioritized in a higher-priority queue, while regular tasks are placed in a lower-priority queue. Within each queue, tasks are sorted based on their weighted runtime: tasks that have higher weight (priority) or use the CPU for less time (smaller runtime) are scheduled sooner, due to their a higher position in the queue.
Moreover, each task gets a time slice budget. When a task is dispatched, it receives a time slice equivalent to the remaining unused portion of its previously allocated time slice (with a minimum threshold applied). This gives latency-sensitive workloads more chances to exceed their time slice when needed to perform short bursts of CPU activity without being interrupted (i.e., real-time audio encoding / decoding workloads).
Typical Use Case
Interactive workloads, such as gaming, live streaming, multimedia, real-time audio encoding/decoding, especially when these workloads are running alongside CPU-intensive background tasks.
In this scenario scx_bpfland ensures that interactive workloads maintain a high level of responsiveness.
Production Ready?
The scheduler is based on scx_rustland, implementing nearly the same scheduling algorithm with minor changes and optimizations to be fully implemented in BPF.
Given that the scx_rustland scheduling algorithm has been extensively tested, this scheduler can be considered ready for production use.