mitosis: preempt on tick and check borrow fairness#3469
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likewhatevs wants to merge 1 commit intosched-ext:mainfrom
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mitosis: preempt on tick and check borrow fairness#3469likewhatevs wants to merge 1 commit intosched-ext:mainfrom
likewhatevs wants to merge 1 commit intosched-ext:mainfrom
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Add ops.tick to zero the running task's slice when this CPU's per-CPU DSQ has waiting work, or when the task is cross-cell and the native cell DSQ has waiting work. This triggers dispatch within the current tick rather than waiting for the full slice to expire, reducing latency for per-CPU DSQ tasks and reclaiming CPUs when native cell work arrives. Add borrow eligibility check in try_pick_idle_cpu: before borrowing a CPU, verify the target cell's DSQ and per-CPU DSQ have no waiting work at borrow time. Work arriving after the borrow starts is handled by tick. Add cpu_dsq_raw() and cell_llc_dsq_raw() helpers to dsq.bpf.h for contexts where BPF does not support aggregate return values from get_cpu_dsq_id() / get_cell_llc_dsq_id(). Signed-off-by: Pat Somaru <patso@likewhatevs.io>
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This was referenced Mar 27, 2026
dschatzberg
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Mar 30, 2026
| target_cctx->cell, tllc)) > 0 || | ||
| scx_bpf_dsq_nr_queued(cpu_dsq_raw(cpu)) > 0) | ||
| goto no_borrow; | ||
| } |
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This doesn't make a ton of sense to me - it seems like papering over some other bug if we're finding idle CPUs in a cell with work in its DSQ
| p->scx.slice = 0; | ||
| } | ||
| } | ||
| } |
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I think I rather make this choice at select_cpu or enqueue time rather than on a periodic tick. That being said - this just seems like a fairness change - maybe let's wait on this until we've resolved more obvious correctness issues?
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Summary
work is waiting, or when a cross-cell borrower occupies a CPU whose
native cell has queued work.
are empty — skip the borrow if the target cell needs the CPU.
return aggregates.
Together with #3468, this gets the vtime contamination test in #3467
to pass.