Re: [PATCH 4/5] sched: Add assertions to QUEUE_CLASS

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 27.11.2025 16:39, Peter Zijlstra wrote:
> Add some checks to the sched_change pattern to validate assumptions
> around changing classes.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>


This patch landed recently in linux-next as commit 47efe2ddccb1 
("sched/core: Add assertions to QUEUE_CLASS"). In my tests it turned out 
that it triggers the following warning during simple 'rtcwake' test on 
Hardkernel's Odroid-M1 board 
(arch/arm64/boot/dts/rockchip/rk3568-odroid-m1.dts):

root@target:~# time rtcwake -s5 -mon
rtcwake: wakeup using /dev/rtc0 at Thu Dec 18 10:01:28 2025
------------[ cut here ]------------
WARNING: kernel/sched/core.c:10837 at sched_change_end+0x160/0x168, 
CPU#0: irq/38-rk817/79
Modules linked in: snd_soc_hdmi_codec dw_hdmi_i2s_audio dw_hdmi_cec 
snd_soc_simple_card snd_soc_rk817 snd_soc_simple_card_utils 
snd_soc_rockchip_i2s_tdm snd_soc_core hantro_vpu rockchip_rga v4l2_vp9 
v4l2_h264 snd_compress v4l2_jpeg videobuf2_dma_sg videobuf2_dma_contig 
v4l2_mem2mem videobuf2_memops snd_pcm_dmaengine videobuf2_v4l2 snd_pcm 
gpio_ir_recv dwmac_rk display_connector stmmac_platform rockchip_saradc 
rockchipdrm snd_timer videodev snd stmmac industrialio_triggered_buffer 
kfifo_buf rockchip_thermal phy_rockchip_naneng_combphy videobuf2_common 
spi_rockchip_sfc soundcore rk817_charger rockchip_dfi rtc_rk808 
rk805_pwrkey pcs_xpcs panfrost dw_hdmi_qp analogix_dp dw_dp 
drm_shmem_helper dw_mipi_dsi drm_dp_aux_bus gpu_sched dw_hdmi mc 
drm_display_helper ahci_dwc ipv6 libsha1
CPU: 0 UID: 0 PID: 79 Comm: irq/38-rk817 Not tainted 6.19.0-rc1+ #16288 
PREEMPT
Hardware name: Hardkernel ODROID-M1 (DT)
pstate: 404000c9 (nZcv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : sched_change_end+0x160/0x168
lr : sched_change_end+0xb0/0x168
...
Call trace:
  sched_change_end+0x160/0x168 (P)
  rt_mutex_setprio+0xc8/0x3a8
  mark_wakeup_next_waiter+0xc0/0x258
  rt_mutex_unlock+0x88/0x148
  i2c_adapter_unlock_bus+0x14/0x20
  i2c_transfer+0xac/0xf0
  regmap_i2c_read+0x5c/0xa0
  _regmap_raw_read+0xec/0x16c
  _regmap_bus_read+0x44/0x7c
  _regmap_read+0x64/0xf4
  regmap_read+0x4c/0x78
  read_irq_data+0x9c/0x460
  regmap_irq_thread+0x64/0x2f0
  irq_thread_fn+0x2c/0xa8
  irq_thread+0x1a4/0x378
  kthread+0x13c/0x214
  ret_from_fork+0x10/0x20
---[ end trace 0000000000000000 ]---

real    0m5.547s
user    0m0.004s
sys     0m0.011s
root@target:~#

I don't see anything suspicious in this stacktrace. Let me know how I 
can help debugging this issue. This board is the only one in my test 
farm which triggers such warning.


> ---
>   kernel/sched/core.c  |   13 +++++++++++++
>   kernel/sched/sched.h |    1 +
>   2 files changed, 14 insertions(+)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10806,6 +10806,7 @@ struct sched_change_ctx *sched_change_be
>   
>   	*ctx = (struct sched_change_ctx){
>   		.p = p,
> +		.class = p->sched_class,
>   		.flags = flags,
>   		.queued = task_on_rq_queued(p),
>   		.running = task_current_donor(rq, p),
> @@ -10836,6 +10837,11 @@ void sched_change_end(struct sched_chang
>   
>   	lockdep_assert_rq_held(rq);
>   
> +	/*
> +	 * Changing class without *QUEUE_CLASS is bad.
> +	 */
> +	WARN_ON_ONCE(p->sched_class != ctx->class && !(ctx->flags & ENQUEUE_CLASS));
> +
>   	if ((ctx->flags & ENQUEUE_CLASS) && p->sched_class->switching_to)
>   		p->sched_class->switching_to(rq, p);
>   
> @@ -10847,6 +10853,13 @@ void sched_change_end(struct sched_chang
>   	if (ctx->flags & ENQUEUE_CLASS) {
>   		if (p->sched_class->switched_to)
>   			p->sched_class->switched_to(rq, p);
> +
> +		/*
> +		 * 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);
>   	}
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -4027,6 +4027,7 @@ extern void balance_callbacks(struct rq
>   struct sched_change_ctx {
>   	u64			prio;
>   	struct task_struct	*p;
> +	const struct sched_class *class;
>   	int			flags;
>   	bool			queued;
>   	bool			running;
>
>
>
Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland