Re: [PATCH 06/10] sched_ext: Delegate proxy donor admission to BPF schedulers

[John Stultz]

On Fri, Jul 10, 2026 at 1:40 AM Andrea Righi <arighi@xxxxxxxxxx> wrote:
> @@ -2866,18 +2938,12 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
>                 set_task_runnable(rq, p);
>
>                 /*
> -                * Mutex-blocked donors stay queued on the runqueue under proxy
> -                * execution, but the donor never runs as itself, proxy-exec
> -                * walks the blocked_on chain on the next __schedule() and runs
> -                * the lock owner in its place.
> -                *
> -                * Put the donor on the local DSQ directly so pick_next_task()
> -                * can still see it. find_proxy_task() will either run the chain
> -                * owner or deactivate the donor so the wakeup path can return it
> -                * and let BPF make a new dispatch decision once it is unblocked.
> +                * Mutex-blocked donors only stay queued when their BPF scheduler
> +                * enables %SCX_OPS_ENQ_BLOCKED, so always delegate their admission.
>                  */
>                 if (task_is_blocked(p)) {
> -                       scx_dispatch_enqueue(sch, rq, &rq->scx.local_dsq, p, 0);
> +                       WARN_ON_ONCE(!(sch->ops.flags & SCX_OPS_ENQ_BLOCKED));
> +                       scx_do_enqueue_task(rq, p, 0, -1);
>                         goto switch_class;
>                 }
>

Just FYI: So in doing some testing running my priority-inversion-demo
test in a loop with this series (rebased on 7.2-rc2), goining through
the example schedulers in tools/sched_ext/ I hit this WARN_ON after
running awhile with the scx_pair scheduler.

Haven't done much debugging on it yet, may have to wait until monday,
but the splat is below.

thanks
-john

[25733.271828] WARNING: kernel/sched/ext/ext.c:2945 at
put_prev_task_scx+0x449/0x460, CPU#0: foreground/25080
[25733.277072] CPU: 0 UID: 0 PID: 25080 Comm: foreground Not tainted
7.2.0-rc2-00043-g9699f4add4b0 #39 PREEMPT(full)
[25733.286458] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.17.0-debian-1.17.0-1 04/01/2014
[25733.295933] Sched_ext: pair (enabled+all), task: runnable_at=-798ms
[25733.295937] RIP: 0010:put_prev_task_scx+0x449/0x460
[25733.306251] Code: 48 8b 83 68 11 00 00 48 8b b5 10 04 00 00 48 39
b0 98 11 00 00 0f 85 79 ff ff ff e9 65 ff ff ff 90 0f 0b 90 e9 46 fe
ff ff 90 <0f> 0b 90 e9 b5 fd ff ff b8 02 00 00 00 e9 94 fc ff ff e8 90
bb 2d
[25733.322497] RSP: 0018:ffffc90011f7faa8 EFLAGS: 00010046
[25733.325750] RAX: 0000000000000001 RBX: ffff8881b8e2db80 RCX: ffff8880bd482650
[25733.333460] RDX: ffff8881b8e2e618 RSI: ffff8880bd482650 RDI: ffff8881073b2650
[25733.338640] RBP: ffff8881073b22c0 R08: 0000000000000000 R09: 0000000000000000
[25733.344710] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88800ba23000
[25733.351179] R13: ffff8880bd4822c0 R14: ffffffff82f6a518 R15: ffffffff82f6a040
[25733.356585] FS:  00007f9badd90780(0000) GS:ffff888234841000(0000)
knlGS:0000000000000000
[25733.362523] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[25733.365426] CR2: 00007f91f1e48110 CR3: 000000001113c006 CR4: 0000000000370ef0
[25733.372285] Call Trace:
[25733.373817]  <TASK>
[25733.375178]  __pick_next_task+0xa8/0x180
[25733.378968]  __schedule+0x268/0x2600
[25733.381092]  ? mark_held_locks+0x40/0x70
[25733.383556]  preempt_schedule_notrace+0x43/0xa0
[25733.386311]  ? preempt_schedule_notrace_thunk+0x16/0x30
[25733.390954]  preempt_schedule_notrace_thunk+0x16/0x30
[25733.395476]  rcu_is_watching+0x3e/0x50
[25733.397722]  lock_acquire+0x2c0/0x310
[25733.401168]  ? _raw_spin_unlock_irqrestore+0x50/0x60
[25733.404224]  schedule+0x9d/0x100
[25733.406428]  ? schedule+0x7e/0x100
[25733.408130]  schedule_preempt_disabled+0x18/0x30
[25733.411008]  __mutex_lock+0x800/0x1100
[25733.415788]  ? __start_renaming+0x5d/0x190
[25733.419011]  ? lock_acquire+0xd9/0x310
[25733.421518]  ? __start_renaming+0x5d/0x190
[25733.424188]  __start_renaming+0x5d/0x190
[25733.426676]  filename_renameat2+0x318/0x420
[25733.429258]  ? find_held_lock+0x2b/0x80
[25733.432581]  __x64_sys_rename+0x48/0x70
[25733.435240]  do_syscall_64+0xf3/0x6c0
[25733.437675]  entry_SYSCALL_64_after_hwframe+0x77/0x7f