Re: [PATCH sched/core] sched/rt: Fix RT_PUSH_IPI soft lockup loop

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 13 May 2026 09:39:14 -1000
Tejun Heo <tj@kernel.org> wrote:

> So, here's a capture from a synthetic reproducer that I think
> models the dynamic and reaches the same end state.

Synthetic capture is fine.

> 
> Test box, 192 CPUs, kernel without the fix:
> 
> - Per-target hrtimer (HRTIMER_MODE_REL_PINNED_HARD) fires every
>   750us. Each fire schedules one tasklet round-robin from a pool
>   of 20k distinct tasklets. Each tasklet body is a 500us cpu_relax
>   loop, standing in for "process one item of softirq work".

So you are running a softirq for 500us every 750us?

This basically prevents any task from running on these CPUs while the
softirq is executing.

> 
> - Storm driver: 190 SCHED_FIFO-50 nanosleep loops on non-target
>   CPUs drive tell_cpu_to_push from balance_rt. Two synthetic
>   psimon-shaped kthreads (FIFO 1) bound to the targets to pin
>   them into rto_mask.

What exactly are these synthetic kthreads doing. Have code to share?

> 
> Baseline (no storm helpers): ~85% softirq util, no lockup, runs
> indefinitely. The reproducer's baseline is higher than production -
> my guess is we need to scrape up against capacity to grow a backlog
> with the fixed-shape workload here, while production gets the same
> effect from bursty arrivals during brief slowdowns.
> 
> With the storm: walker IPI overhead stretches each tasklet body
> from 500us to ~1.1ms. Service rate drops below arrival, backlog
> grows ~430/s. After ~46s, one tasklet_action_common snapshot has
> ~20k tasklets which it processes serially in BH-disabled softirq
> context. That's ~22s uninterruptible, watchdog fires.

The IPI walker should only go to the CPUs with overloaded RT tasks. Are you
making all the CPUS have overloaded RT tasks?

> 
> Six soft-lockups in a 120s run:
> 
>   [61125.38] BUG: soft lockup - CPU#95 stuck for 22s! [kworker/95:0]
>   [61145.38] BUG: soft lockup - CPU#47 stuck for 45s! [migration/47]
>   [61173.38] BUG: soft lockup - CPU#47 stuck for 71s! [migration/47]
>   [61197.38] BUG: soft lockup - CPU#95 stuck for 22s! [migration/95]
>   [61209.38] BUG: soft lockup - CPU#47 stuck for 21s! [kworker/47:1]
>   [61225.38] BUG: soft lockup - CPU#95 stuck for 48s! [migration/95]
> 
> Stack at fire:
> 
>   rt_storm_wedge_fn+0x22/0xe0
>   tasklet_action_common+0x100/0x2b0
>   handle_softirqs+0xbe/0x280
>   __irq_exit_rcu+0x47/0x100
>   sysvec_apic_timer_interrupt+0x3a/0x80     <- watchdog hrtimer
>   asm_sysvec_apic_timer_interrupt+0x16/0x20
>   RIP: 0033:0x...     <- user task (rt_storm_hog)
> 
> Trace captured with your event list plus IPI:
> 
>   -e sched_switch -e sched_waking -e irq -e workqueue -e ipi
>   -e irq_vectors:call_function_single_entry/exit
>   -e irq_vectors:irq_work_entry/exit
>   -e irq_vectors:reschedule_entry/exit
>   -e irq_vectors:local_timer_entry/exit
> 
> Sliced to a 17s window around the first RCU stall + first
> soft-lockup, filtered to CPUs 47 and 95, gzipped text (~11MB):
> 
>   https://drive.google.com/file/d/11AN6dyvOWiZLVNEEuVtQieRyAxJYbCbt/view?usp=sharing
> 

So this is showing that the IPI logic is just extending the softirq work
load to something greater than the period of execution and causing a live
lock of softirqs.

This still doesn't explain to me why the current process is of a lower
priority than a waiting RT task.

I'm really starting to think you are fixing a symptom and not the cause.

-- Steve