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A task's slice grants it cpu occupancy - how long it holds its cpu. In a sub-scheduler hierarchy cpu access is delegated through revocable capabilities, so a task's occupancy must follow them. Only its own scheduler sets its slice, and extending the slice is allowed only while that scheduler holds baseline cpu access (SCX_CAP_BASE) on the cpu. Otherwise a scheduler could keep occupying a cpu it has been denied simply by handing out long slices. The cap check reads effective caps, which are coherent only under the task's rq lock, and the kernel decrements the slice under that lock as the task runs, so a running task's slice can be changed only there while a queued task's can be set directly. Make scx_bpf_task_set_slice() apply the slice under the rq lock. Synchronously when the caller already holds it, otherwise by stashing it in the new p->scx.slice_oob, tagged with the scheduler's id so a request that outlived a reassignment is dropped. Whether the caller holds @p's current rq lock is tested with p->scx.runnable_cpu. Revocation is enforced through the same grant. When a cpu's effective caps lose SCX_CAP_BASE, the cap-revoke reenq scan also checks the running task and zeroes its slice to evict it. The scan runs as a balance callback after the pick, so this catches both the task that was running when the revoke landed and a capless task the pick just promoted off the local DSQ. The paths that keep a task on its cpu - holding on to the last runnable task in balance, the ENQ_LAST reinsertion and the slice refill on pick - skip tasks lacking baseline access. A migration-disabled task is exempt, mirroring its capless admission on insert. v4: Test rq ownership with p->scx.runnable_cpu, closing a remote-wakeup TOCTOU. (sashiko AI) v3: Keep a pending out-of-band slice request across refill and preserve. (sashiko AI) v2: Only write slice directly when @p is queued on the held rq. (sashiko AI) Signed-off-by: Tejun Heo <tj@kernel.org> Reviewed-by: Andrea Righi <arighi@nvidia.com> |
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