Re: [RFC PATCH v2 0/7] Defer throttle when task exits to user

K Prateek Nayak
2025-04-15T08:45:15Z

(+ Sebastian)

Hello Jan,

On 4/15/2025 11:39 AM, Jan Kiszka wrote:
>>> Attached the bits with which we succeeded, sometimes. Setup: Debian 12,
>>> RT kernel, 2-4 cores VM, 1-5 instances of the test, 2 min - 2 h
>>> patience. As we have to succeed with at least 3 race conditions in a
>>> row, that is still not bad... But maybe someone has an idea how to
>>> increase probabilities further.
>>
>> Looking at run.sh, there are only fair tasks with one of them being run
>> with cfs bandwidth constraints. Are you saying something goes wrong on
>> PREEMPT_RT as a result of using bandwidth control on fair tasks?
>
> Yes, exactly. Also our in-field workload that triggers (most likely)
> this issue is not using RT tasks itself. Only kernel threads are RT here.
>
>>
>> What exactly is the symptom you are observing? Does one of the assert()
>> trip during the run? Do you see a stall logged on dmesg? Can you provide
>> more information on what to expect in this 2min - 2hr window?
>
> I've just lost my traces from yesterday ("you have 0 minutes to find a
> power adapter"), but I got nice RCU stall warnings in the VM, including
> backtraces from the involved tasks (minus the read-lock holder IIRC).
> Maybe Florian can drop one of his dumps.

So I ran your reproducer on a 2vCPU VM running v6.15-rc1 PREEMPT_RT
and I saw:

rcu: INFO: rcu_preempt self-detected stall on CPU
rcu: 0-...!: (15000 ticks this GP) idle=8a74/0/0x1 softirq=0/0 fqs=0
rcu: (t=15001 jiffies g=12713 q=24 ncpus=2)
rcu: rcu_preempt kthread timer wakeup didn't happen for 15000 jiffies! g12713 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
rcu: Possible timer handling issue on cpu=0 timer-softirq=17688
rcu: rcu_preempt kthread starved for 15001 jiffies! g12713 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 ->cpu=0
rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
rcu: RCU grace-period kthread stack dump:
task:rcu_preempt state:I stack:0 pid:17 tgid:17 ppid:2 task_flags:0x208040 flags:0x00004000
Call Trace:
<TASK>
__schedule+0x401/0x15a0
? srso_alias_return_thunk+0x5/0xfbef5
? lock_timer_base+0x77/0xb0
? srso_alias_return_thunk+0x5/0xfbef5
? __pfx_rcu_gp_kthread+0x10/0x10
schedule+0x27/0xd0
schedule_timeout+0x76/0x100
? __pfx_process_timeout+0x10/0x10
rcu_gp_fqs_loop+0x10a/0x4b0
rcu_gp_kthread+0xd3/0x160
kthread+0xff/0x210
? rt_spin_lock+0x3c/0xc0
? __pfx_kthread+0x10/0x10
ret_from_fork+0x34/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30
</TASK>
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.15.0-rc1-test-dirty #746 PREEMPT_{RT,(full)}
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
RIP: 0010:pv_native_safe_halt+0xf/0x20
Code: 22 df e9 1f 08 e5 fe 0f 1f 40 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa eb 07 0f 00 2d 85 96 15 00 fb f4 <e9> f7 07 e5 fe 66 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90
RSP: 0018:ffffffff95803e50 EFLAGS: 00000216
RAX: ffff8e2d61534000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000081f8a6c
RBP: ffffffff9581d280 R08: 0000000000000000 R09: ffff8e2cf7d32301
R10: ffff8e2be11ae5c8 R11: 0000000000000001 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 00000000000147b0
FS: 0000000000000000(0000) GS:ffff8e2d61534000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055e77c3a5128 CR3: 000000010ff78003 CR4: 0000000000770ef0
PKRU: 55555554
Call Trace:
<TASK>
default_idle+0x9/0x20
default_idle_call+0x30/0x100
do_idle+0x20f/0x250
? do_idle+0xb/0x250
cpu_startup_entry+0x29/0x30
rest_init+0xde/0x100
start_kernel+0x733/0xb20
? copy_bootdata+0x9/0xb0
x86_64_start_reservations+0x18/0x30
x86_64_start_kernel+0xba/0x110
common_startup_64+0x13e/0x141
</TASK>

Is this in line with what you are seeing?

>
>>
>> Additionally, do you have RT throttling enabled in your setup? Can long
>> running RT tasks starve fair tasks on your setup?
>
> RT throttling is enabled (default settings) but was not kicking in - why
> should it in that scenario? The only RT thread, ktimers, ran into the
> held lock and stopped.
>
> Jan
>

--
Thanks and Regards,
Prateek