scx_mitosis: adaptive stealing, orphan-DSQ drain, migration tracking#3572
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scx_mitosis: adaptive stealing, orphan-DSQ drain, migration tracking#3572tommy-u wants to merge 1 commit into
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1) Adaptive stealing controller. Per-cell sibling-LLC queue-depth
threshold (per_cell_steal_min_queued[]) tuned by userspace from the
observed steal-success ratio. try_stealing_work picks the most
backed-up sibling LLC and only steals if its depth exceeds the
threshold, throttling cross-LLC churn under low pressure.
2) Drain for orphaned tasks. When apply_cell_config or a cgroup
change shrinks a cell out of an LLC, recalc_cell_llc_counts now
publishes the new per-LLC cpu_cnt and then iterates the stranded
(cell, LLC) DSQ, moving each task to a sibling LLC's DSQ that the
cell still owns CPUs in. Caller order is publish_prepared_cpumask
first, then recalc, so re-pickers triggered by the cpu_cnt==0 guard
see the new cell membership.
3) Migration source tracking. New last_cpu_source field on task_ctx
plus 2x3 cross-product counters (same|cross)x(select|enqueue|
dispatch). track_cpu_migration in mitosis_running attributes each
migration to whichever path placed the task. Stolen-task tagging
collapses into maybe_retag_stolen_task (LLC mismatch is the unique
steal signal at running time), removing the per-CPU stolen_dispatch
flag and its dispatch-to-running race window.
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Adaptive stealing controller. Per-cell sibling-LLC queue-depth
threshold (per_cell_steal_min_queued[]) tuned by userspace from the
observed steal-success ratio. try_stealing_work picks the most
backed-up sibling LLC and only steals if its depth exceeds the
threshold, throttling cross-LLC churn under low pressure.
Drain for orphaned tasks. When apply_cell_config or a cgroup
change shrinks a cell out of an LLC, recalc_cell_llc_counts now
publishes the new per-LLC cpu_cnt and then iterates the stranded
(cell, LLC) DSQ, moving each task to a sibling LLC's DSQ that the
cell still owns CPUs in. Caller order is publish_prepared_cpumask
first, then recalc, so re-pickers triggered by the cpu_cnt==0 guard
see the new cell membership.
Migration source tracking. New last_cpu_source field on task_ctx
plus 2x3 cross-product counters (same|cross)x(select|enqueue|
dispatch). track_cpu_migration in mitosis_running attributes each
migration to whichever path placed the task. Stolen-task tagging
collapses into maybe_retag_stolen_task (LLC mismatch is the unique
steal signal at running time), removing the per-CPU stolen_dispatch
flag and its dispatch-to-running race window.