Commit e7a6395a authored Jul 09, 2024 by Tejun Heo

sched_ext: Make scx_bpf_reenqueue_local() skip tasks that are being migrated



When a running task is migrated to another CPU, the stop_task is used to
preempt the running task and migrate it. This, expectedly, invokes
ops.cpu_release(). If the BPF scheduler then calls
scx_bpf_reenqueue_local(), it re-enqueues all tasks on the local DSQ
including the task which is being migrated.

This creates an unnecessary re-enqueue of a task which is about to be
deactivated and re-activated for migration anyway. It can also cause
confusion for the BPF scheduler as scx_bpf_task_cpu() of the task and its
allowed CPUs may not agree while migration is pending.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 245254f7 ("sched_ext: Implement sched_ext_ops.cpu_acquire/release()")
Acked-by: David Vernet <void@manifault.com>

parent fd0cf516

kernel/sched/ext.c

+14 −0

Original line number	Diff line number	Diff line
		@@ -5646,6 +5646,20 @@ __bpf_kfunc u32 scx_bpf_reenqueue_local(void)
		*/
		list_for_each_entry_safe(p, n, &rq->scx.local_dsq.list,
		scx.dsq_list.node) {
		/*
		* If @p is being migrated, @p's current CPU may not agree with
		* its allowed CPUs and the migration_cpu_stop is about to
		* deactivate and re-activate @p anyway. Skip re-enqueueing.
		*
		* While racing sched property changes may also dequeue and
		* re-enqueue a migrating task while its current CPU and allowed
		* CPUs disagree, they use %ENQUEUE_RESTORE which is bypassed to
		* the current local DSQ for running tasks and thus are not
		* visible to the BPF scheduler.
		*/
		if (p->migration_pending)
		continue;

		dispatch_dequeue(rq, p);
		list_add_tail(&p->scx.dsq_list.node, &tasks);
		}