linux-kernel - Re: [PATCH sched_ext/for-6.12] sched_ext: TASK_DEAD tasks must be switched out of SCX on ops

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Message-ID: <ZtjBrxdWJzMBjTxW@slm.duckdns.org>
Date: Wed, 4 Sep 2024 10:23:11 -1000
From: Tejun Heo <tj@...nel.org>
To: David Vernet <void@...ifault.com>
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com,
	Andrea Righi <andrea.righi@...ux.dev>
Subject: Re: [PATCH sched_ext/for-6.12] sched_ext: TASK_DEAD tasks must be
 switched out of SCX on ops_disable

On Fri, Aug 30, 2024 at 01:44:40PM -1000, Tejun Heo wrote:
> scx_ops_disable_workfn() only switches !TASK_DEAD tasks out of SCX while
> calling scx_ops_exit_task() on all tasks including dead ones. This can leave
> a dead task on SCX but with SCX_TASK_NONE state, which is inconsistent.
> 
> If another task was in the process of changing the TASK_DEAD task's
> scheduling class and grabs the rq lock after scx_ops_disable_workfn() is
> done with the task, the task ends up calling scx_ops_disable_task() on the
> dead task which is in an inconsistent state triggering a warning:
> 
>   WARNING: CPU: 6 PID: 3316 at kernel/sched/ext.c:3411 scx_ops_disable_task+0x12c/0x160
>   ...
>   RIP: 0010:scx_ops_disable_task+0x12c/0x160
>   ...
>   Call Trace:
>    <TASK>
>    check_class_changed+0x2c/0x70
>    __sched_setscheduler+0x8a0/0xa50
>    do_sched_setscheduler+0x104/0x1c0
>    __x64_sys_sched_setscheduler+0x18/0x30
>    do_syscall_64+0x7b/0x140
>    entry_SYSCALL_64_after_hwframe+0x76/0x7e
>   RIP: 0033:0x7f140d70ea5b
> 
> There is no reason to leave dead tasks on SCX when unloading the BPF
> scheduler. Fix by making scx_ops_disable_workfn() eject all tasks including
> the dead ones from SCX.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>

Applied to sched_ext/for-6.12.

Thanks.

-- 
tejun