tjh.dev / kernel
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sched_ext: Fix missing post-enqueue handling in move_local_task_to_local_dsq()

move_local_task_to_local_dsq() is used when moving a task from a non-local
DSQ to a local DSQ on the same CPU. It directly manipulates the local DSQ
without going through dispatch_enqueue() and was missing the post-enqueue
handling that triggers preemption when SCX_ENQ_PREEMPT is set or the idle
task is running.

The function is used by move_task_between_dsqs() which backs
scx_bpf_dsq_move() and may be called while the CPU is busy.

Add local_dsq_post_enq() call to move_local_task_to_local_dsq(). As the
dispatch path doesn't need post-enqueue handling, add SCX_RQ_IN_BALANCE
early exit to keep consume_dispatch_q() behavior unchanged and avoid
triggering unnecessary resched when scx_bpf_dsq_move() is used from the
dispatch path.

Fixes: 4c30f5ce4f7a ("sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()")
Cc: stable@vger.kernel.org # v6.12+
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Tejun Heo f5e1e5ec 530b6637

+10
+10
kernel/sched/ext.c
··· 988 988 struct rq *rq = container_of(dsq, struct rq, scx.local_dsq); 989 989 bool preempt = false; 990 990 991 + /* 992 + * If @rq is in balance, the CPU is already vacant and looking for the 993 + * next task to run. No need to preempt or trigger resched after moving 994 + * @p into its local DSQ. 995 + */ 996 + if (rq->scx.flags & SCX_RQ_IN_BALANCE) 997 + return; 998 + 991 999 if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr && 992 1000 rq->curr->sched_class == &ext_sched_class) { 993 1001 rq->curr->scx.slice = 0; ··· 1644 1636 1645 1637 dsq_mod_nr(dst_dsq, 1); 1646 1638 p->scx.dsq = dst_dsq; 1639 + 1640 + local_dsq_post_enq(dst_dsq, p, enq_flags); 1647 1641 } 1648 1642 1649 1643 /**