linux-kernel - Re: [PATCH v2] vmstat: disable vmstat_work on vmstat_cpu_down

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Message-ID: <Z3uqu0ue0agUASzz@J2N7QTR9R3>
Date: Mon, 6 Jan 2025 10:04:43 +0000
From: Mark Rutland <mark.rutland@....com>
To: Koichiro Den <koichiro.den@...onical.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, linux-mm@...ck.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vmstat: disable vmstat_work on vmstat_cpu_down_prep()

On Sat, Jan 04, 2025 at 01:00:17PM +0900, Koichiro Den wrote:
> On Fri, Jan 03, 2025 at 11:33:19PM +0000, Lorenzo Stoakes wrote:
> > Hi,
> > 
> > I observed a warning in my qemu and real hardware, which I bisected to this commit:
> > 
> > [    0.087733] ------------[ cut here ]------------
> > [    0.087733] workqueue: work disable count underflowed
> > [    0.087733] WARNING: CPU: 1 PID: 21 at kernel/workqueue.c:4313 enable_work+0xb5/0xc0

FWIW, I hit similar when testing v6.13-rc6 defconfig on arm64, when
booting secondaries I always get a splat (trimmed):

| ------------[ cut here ]------------
| workqueue: work disable count underflowed
| WARNING: CPU: 1 PID: 21 at kernel/workqueue.c:4317 enable_work+0xfc/0x108
| Modules linked in:
| CPU: 1 UID: 0 PID: 21 Comm: cpuhp/1 Not tainted 6.13.0-rc6 #1
| Hardware name: linux,dummy-virt (DT)
| [...]
| Call trace:
|  enable_work+0xfc/0x108 (P)
|  enable_delayed_work+0x10/0x1c
|  vmstat_cpu_online+0x88/0xbc
|  cpuhp_invoke_callback+0x10c/0x208
|  cpuhp_thread_fun+0xb0/0x1a0
|  smpboot_thread_fn+0x20c/0x234
|  kthread+0x110/0x114
|  ret_from_fork+0x10/0x20
| ---[ end trace 0000000000000000 ]---

[...]

> In my current view, the simplest solution would be to make sure a local
> vmstat_work is disabled until vmstat_cpu_online() runs for the cpu, even
> during boot-up. The following patch suppresses the warning:
> 
>   diff --git a/mm/vmstat.c b/mm/vmstat.c
>   index 0889b75cef14..19ceed5d34bf 100644
>   --- a/mm/vmstat.c
>   +++ b/mm/vmstat.c
>   @@ -2122,10 +2122,14 @@ static void __init start_shepherd_timer(void)
>    {
>           int cpu;
>   
>   -       for_each_possible_cpu(cpu)
>   +       for_each_possible_cpu(cpu) {
>                   INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu),
>                           vmstat_update);
>   
>   +               /* will be enabled on vmstat_cpu_online */
>   +               disable_delayed_work_sync(&per_cpu(vmstat_work, cpu));
>   +       }
>   +
>           schedule_delayed_work(&shepherd,
>                   round_jiffies_relative(sysctl_stat_interval));
>    }

FWIW, the above solves the warning for me.

Mark.