Re: [PATCH sched_ext/for-6.14-fixes 2/2] sched_ext: Fix migration disabled handling in targeted dispatches

[Tejun Heo <tj@kernel.org>]

On Fri, Feb 07, 2025 at 10:59:06AM -1000, Tejun Heo wrote:
> A dispatch operation that can target a specific local DSQ -
> scx_bpf_dsq_move_to_local() or scx_bpf_dsq_move() - checks whether the task
> can be migrated to the target CPU using task_can_run_on_remote_rq(). If the
> task can't be migrated to the targeted CPU, it is bounced through a global
> DSQ.
> 
> task_can_run_on_remote_rq() assumes that the task is on a CPU that's
> different from the targeted CPU but the callers doesn't uphold the
> assumption and may call the function when the task is already on the target
> CPU. When such task has migration disabled, task_can_run_on_remote_rq() ends
> up returning %false incorrectly unnecessarily bouncing the task to a global
> DSQ.
> 
> Fix it by updating the callers to only call task_can_run_on_remote_rq() when
> the task is on a different CPU than the target CPU. As this is a bit subtle,
> for clarity and documentation:
> 
> - Make task_can_run_on_remote_rq() trigger SCHED_WARN_ON() if the task is on
>   the same CPU as the target CPU.
> 
> - is_migration_disabled() test in task_can_run_on_remote_rq() cannot trigger
>   if the task is on a different CPU than the target CPU as the preceding
>   task_allowed_on_cpu() test should fail beforehand. Convert the test into
>   SCHED_WARN_ON().
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Fixes: 4c30f5ce4f7a ("sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()")
> Fixes: 0366017e0973 ("sched_ext: Use task_can_run_on_remote_rq() test in dispatch_to_local_dsq()")
> Cc: stable@vger.kernel.org # v6.12+

Applied 1-2 to sched_ext/for-6.14-fixes.

Thanks.

-- 
tejun