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The ops.cpu_acquire/release() callbacks miss events under multiple conditions. There are two distinct task dispatch gaps that can cause cpu_released flag desynchronization: 1. balance-to-pick_task gap: This is what was originally reported. balance_scx() can enqueue a task, but during consume_remote_task() when the rq lock is released, a higher priority task can be enqueued and ultimately picked while cpu_released remains false. This gap is closeable via RETRY_TASK handling. 2. ttwu-to-pick_task gap: ttwu() can directly dispatch a task to a CPU's local DSQ. By the time the sched path runs on the target CPU, higher class tasks may already be queued. In such cases, nothing on sched_ext side will be invoked, and the only solution would be a hook invoked regardless of sched class, which isn't desirable. Rather than adding invasive core hooks, BPF schedulers can use generic BPF mechanisms like tracepoints. From SCX scheduler's perspective, this is congruent with other mechanisms it already uses and doesn't add further friction. The main use case for cpu_release() was calling scx_bpf_reenqueue_local() when a CPU gets preempted by a higher priority scheduling class. However, the old scx_bpf_reenqueue_local() could only be called from cpu_release() context. Add a new version of scx_bpf_reenqueue_local() that can be called from any context by deferring the actual re-enqueue operation. This eliminates the need for cpu_acquire/release() ops entirely. Schedulers can now use standard BPF mechanisms like the sched_switch tracepoint to detect and handle CPU preemption. Update scx_qmap to demonstrate the new approach using sched_switch instead of cpu_release, with compat support for older kernels. Mark cpu_acquire/release() as deprecated. The old scx_bpf_reenqueue_local() variant will be removed in v6.23. Reported-by: Wen-Fang Liu <liuwenfang@honor.com> Link: https://lore.kernel.org/all/8d64c74118c6440f81bcf5a4ac6b9f00@honor.com/ Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Tejun Heo <tj@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.