Re: [PATCH] sched/ext: Fix invalid task state transitions on class switch

[Tejun Heo <tj@kernel.org>]

On Tue, Aug 05, 2025 at 10:59:11AM +0200, Andrea Righi wrote:
> When enabling a sched_ext scheduler, we may trigger invalid task state
> transitions, resulting in warnings like the following (which can be
> easily reproduced by running the hotplug selftest in a loop):
> 
>  sched_ext: Invalid task state transition 0 -> 3 for fish[770]
>  WARNING: CPU: 18 PID: 787 at kernel/sched/ext.c:3862 scx_set_task_state+0x7c/0xc0
>  ...
>  RIP: 0010:scx_set_task_state+0x7c/0xc0
>  ...
>  Call Trace:
>   <TASK>
>   scx_enable_task+0x11f/0x2e0
>   switching_to_scx+0x24/0x110
>   scx_enable.isra.0+0xd14/0x13d0
>   bpf_struct_ops_link_create+0x136/0x1a0
>   __sys_bpf+0x1edd/0x2c30
>   __x64_sys_bpf+0x21/0x30
>   do_syscall_64+0xbb/0x370
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> This happens because we skip initialization for tasks that are already
> dead (with their usage counter set to zero), but we don't exclude them
> during the scheduling class transition phase.
> 
> Fix this by also skipping dead tasks during class swiching, preventing
> invalid task state transitions.
> 
> Fixes: a8532fac7b5d2 ("sched_ext: TASK_DEAD tasks must be switched into SCX on ops_enable")
> Signed-off-by: Andrea Righi <arighi@nvidia.com>

Applied to sched_ext/for-6.17-fixes.

Thanks.

-- 
tejun