On Tue, Dec 16, 2025 at 06:11:11PM +0530, Naresh Kamboju wrote:

[Re-sending with unwrapping]

The following boot warning is noticed on qemu-arm64 booting the Linux next-20251215 and next-20251216.

Regression Analysis:

• New regression? Yes
• Reproducibility? Yes

First seen on next-20251215 Bad: next-20251215 and next-20251216 Good: next-20251212

Boot regression: arm64: WARNING: kernel/sched/core.c:10851 at sched_change_end

Are these expected warnings ? Anyone noticed ?

Reported-by: Linux Kernel Functional Testing lkft@linaro.org

Still a wrapped mess :/

Anyway, afaik you're the first to notice.

Full boot log links provided at the bottom of the email.

## Boot log [ 0.000000] Linux version 6.19.0-rc1-next-20251215 (tuxmake@tuxmake) (aarch64-linux-gnu-gcc (Debian 13.3.0-16) 13.3.0, GNU ld (GNU Binutils for Debian) 2.44) #1 SMP PREEMPT @1765798467 ... [ 14.659447] mlx5_core 0000:01:00.1 enp1s0f1np1: renamed from eth1 [ 14.691267] CPPC Cpufreq:FIE not enabled on systems with registers in PCC ** replaying previous printk message ** [ 14.691267] CPPC Cpufreq:FIE not enabled on systems with registers in PCC [ 14.694920] mlx5_core 0000:01:00.0 enp1s0f0np0: renamed from eth0 (while UP) [ 14.696414] ------------[ cut here ]------------ [ 14.696418] WARNING: kernel/sched/core.c:10851 at sched_change_end+0x168/0x188, CPU#12: ktimers/12/117 [ 14.729321] Modules linked in: cppc_cpufreq(+) arm_dsu_pmu(+) fuse drm backlight [ 14.736718] CPU: 12 UID: 0 PID: 117 Comm: ktimers/12 Not tainted 6.19.0-rc1-next-20251216 #1 PREEMPT_RT [ 14.746190] Hardware name: WIWYNN Mt.Jade Server System B81.030Z1.0010/Mt.Jade Motherboard, BIOS 2.10.20250506-1P (SCP: 2.10.20250506) 2025/05/06 [ 14.759217] pstate: 804000c9 (Nzcv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--) [ 14.766169] pc : sched_change_end (kernel/sched/core.c:10851 (discriminator 7)) [ 14.770519] lr : sched_change_end (kernel/sched/core.c:10842 (discriminator 1)) [ 14.774781] sp : ffff800080d33b90 [ 14.778084] x29: ffff800080d33b90 x28: ffff07ff822f5500 x27: ffff80008e0c3660 [ 14.785213] x26: 00000000000000f0 x25: ffff07ff822f5500 x24: 0000000000000000 [ 14.792342] x23: 00000000ffffffff x22: ffffb4099c0d64d0 x21: ffff07ff822f5500 [ 14.799469] x20: ffff083e5eeaf140 x19: ffff083e5ee9b290 x18: 0000000000000000 [ 14.806597] x17: 0000000000000000 x16: ffffb4099a40ee68 x15: 0000000000000000 [ 14.813725] x14: 0000000000000000 x13: 0000000000000000 x12: 0000020000000000 [ 14.820853] x11: 00000000000000c0 x10: 0000000000000c60 x9 : ffffb40999b8d640 [ 14.827981] x8 : 000000000000000c x7 : ffff07ff92ad8400 x6 : 00000000ffffffff [ 14.835109] x5 : 00000000000000ee x4 : 000000036b338800 x3 : 000000036b33b800 [ 14.842238] x2 : 0000000000000001 x1 : ffffb4099c0d63d8 x0 : 0000000000000010 [ 14.849366] Call trace: [ 14.851802] sched_change_end (kernel/sched/core.c:10851 (discriminator 7)) (P) [ 14.856153] rt_mutex_setprio (kernel/sched/sched.h:1813 kernel/sched/core.c:7368) [ 14.860157] rt_mutex_slowunlock (kernel/locking/rtmutex.c:1341 kernel/locking/rtmutex.c:1466) [ 14.864422] rt_spin_unlock (kernel/locking/spinlock_rt.c:86) [ 14.868165] __hrtimer_run_queues (include/linux/spinlock_rt.h:44 kernel/time/hrtimer.c:1398 kernel/time/hrtimer.c:1843) [ 14.872513] hrtimer_run_softirq (kernel/time/hrtimer.c:579 kernel/time/hrtimer.c:1193 kernel/time/hrtimer.c:1861) [ 14.876600] handle_softirqs.isra.0 (arch/arm64/include/asm/jump_label.h:36 include/trace/events/irq.h:142 kernel/softirq.c:627) [ 14.881125] run_ktimerd (kernel/softirq.c:325 kernel/softirq.c:1144) [ 14.884520] smpboot_thread_fn (kernel/smpboot.c:160) [ 14.888608] kthread (kernel/kthread.c:463) [ 14.891828] ret_from_fork (arch/arm64/kernel/entry.S:861) [ 14.895396] ---[ end trace 0000000000000000 ]---

This is:

WARN_ON_ONCE(sched_class_above(ctx->class, p->sched_class) && !test_tsk_need_resched(p));

And rt_mutex_setprio() doesn't set need_resched() on demotion; but worse, reading it now, I realize that if p != current, nobody ever will.

I suppose something like so will cure things?

--- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 7d0a862a8c75..5b17d8e3cb55 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -10833,23 +10833,24 @@ void sched_change_end(struct sched_change_ctx *ctx) if (p->sched_class->switched_to) p->sched_class->switched_to(rq, p);

- /* - * If this was a class promotion; let the old class know it - * got preempted. Note that none of the switch*_from() methods - * know the new class and none of the switch*_to() methods - * know the old class. - */ - if (ctx->running && sched_class_above(p->sched_class, ctx->class)) { - rq->next_class->wakeup_preempt(rq, p, 0); - rq->next_class = p->sched_class; + if (ctx->running) { + /* + * If this was a class promotion; let the old class + * know it got preempted. Note that none of the + * switch*_from() methods know the new class and none + * of the switch*_to() methods know the old class. + */ + if (sched_class_above(p->sched_class, ctx->class)) { + rq->next_class->wakeup_preempt(rq, p, 0); + rq->next_class = p->sched_class; + } + /* + * If this was a degradation in class; make sure to + * reschedule. + */ + if (sched_class_above(ctx->class, p->sched_class)) + resched_curr(rq); } - - /* - * If this was a degradation in class someone should have set - * need_resched by now. - */ - WARN_ON_ONCE(sched_class_above(ctx->class, p->sched_class) && - !test_tsk_need_resched(p)); } else { p->sched_class->prio_changed(rq, p, ctx->prio); }