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

Jan Kiszka
2025-04-15T10:21:24Z

On 15.04.25 10:45, K Prateek Nayak wrote:
> (+ 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?
>

Yes, and if you wait a bit longer for the second reporting round, you
should get more task backtraces as well.

Jan

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