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

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Message-ID: <20250127112455.GG16742@noisy.programming.kicks-ass.net>
Date: Mon, 27 Jan 2025 12:24:55 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com, sched-ext@...a.com,
	David Vernet <void@...ifault.com>, Andrea Righi <arighi@...dia.com>,
	Changwoo Min <changwoo@...lia.com>
Subject: Re: [PATCH sched_ext/for-6.14-fixes] sched_ext: Fix incorrect
 autogroup migration detection

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@...r.kernel.org # v6.12+
> Signed-off-by: Tejun Heo <tj@...nel.org>
> ---
> Peter, I can also flag the task that's doing the cgroup migration but that
> seems unnecessarily convoluted. If you're okay with the change, I'll route
> this through the sched_ext tree.

ACK, Thanks!