Re: [PATCH sched_ext/for-6.14-fixes] sched_ext: Fix incorrect autogroup migration detection

Tejun Heo
2025-01-27T18:33:08Z

On Fri, Jan 24, 2025 at 12:22:12PM -1000, Tejun Heo wrote:
> scx_move_task() is called from sched_move_task() and tells the BPF scheduler
> that cgroup migration is being committed. sched_move_task() is used by both
> cgroup and autogroup migrations and scx_move_task() tried to filter out
> autogroup migrations by testing the destination cgroup and PF_EXITING but
> this is not enough. In fact, without explicitly tagging the thread which is
> doing the cgroup migration, there is no good way to tell apart
> scx_move_task() invocations for racing migration to the root cgroup and an
> autogroup migration.
>
> This led to scx_move_task() incorrectly ignoring a migration from non-root
> cgroup to an autogroup of the root cgroup triggering the following warning:
>
> WARNING: CPU: 7 PID: 1 at kernel/sched/ext.c:3725 scx_cgroup_can_attach+0x196/0x340
> ...
> Call Trace:
> <TASK>
> cgroup_migrate_execute+0x5b1/0x700
> cgroup_attach_task+0x296/0x400
> __cgroup_procs_write+0x128/0x140
> cgroup_procs_write+0x17/0x30
> kernfs_fop_write_iter+0x141/0x1f0
> vfs_write+0x31d/0x4a0
> __x64_sys_write+0x72/0xf0
> do_syscall_64+0x82/0x160
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Fix it by adding an argument to sched_move_task() that indicates whether the
> moving is for a cgroup or autogroup migration. After the change,
> scx_move_task() is called only for cgroup migrations and renamed to
> scx_cgroup_move_task().
>
> Link: https://github.com/sched-ext/scx/issues/370
> Fixes: 819513666966 ("sched_ext: Add cgroup support")
> Cc: stable@vger.kernel.org # v6.12+
> Signed-off-by: Tejun Heo <tj@kernel.org>

Applied to sched_ext/for-6.14-fixes.

Thanks.

--
tejun