linux-kernel - Re: [PATCH v2] kernel/fork: Increase minimum number of allowed threads

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Message-ID: <48e6e92d-6b6a-4850-9396-f3afa327ca3a@kernel.org>
Date: Thu, 17 Jul 2025 07:26:59 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Hauke Mehrtens <hauke@...ke-m.de>, sashal@...nel.org,
 linux-kernel@...r.kernel.org
Cc: frederic@...nel.org, david@...hat.com, viro@...iv.linux.org.uk,
 paulmck@...nel.org, stable@...r.kernel.org,
 David Laight <david.laight.linux@...il.com>, Tejun Heo <tj@...nel.org>,
 Lai Jiangshan <jiangshanlai@...il.com>,
 "Luis R . Rodriguez" <mcgrof@...nel.org>
Subject: Re: [PATCH v2] kernel/fork: Increase minimum number of allowed
 threads

Cc wqueue & umode helper folks

On 12. 07. 25, 1:08, Hauke Mehrtens wrote:
> A modern Linux system creates much more than 20 threads at bootup.
> When I booted up OpenWrt in qemu the system sometimes failed to boot up
> when it wanted to create the 419th thread. The VM had 128MB RAM and the
> calculation in set_max_threads() calculated that max_threads should be
> set to 419. When the system booted up it tried to notify the user space
> about every device it created because CONFIG_UEVENT_HELPER was set and
> used. I counted 1299 calls to call_usermodehelper_setup(), all of
> them try to create a new thread and call the userspace hotplug script in
> it.
> 
> This fixes bootup of Linux on systems with low memory.
> 
> I saw the problem with qemu 10.0.2 using these commands:
> qemu-system-aarch64 -machine virt -cpu cortex-a57 -nographic
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Hauke Mehrtens <hauke@...ke-m.de>
> ---
>   kernel/fork.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 7966c9a1c163..388299525f3c 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -115,7 +115,7 @@
>   /*
>    * Minimum number of threads to boot the kernel
>    */
> -#define MIN_THREADS 20
> +#define MIN_THREADS 600

As David noted, this is not the proper fix. It appears that usermode 
helper should use limited thread pool. I.e. instead of 
system_unbound_wq, alloc_workqueue("", WQ_UNBOUND, max_active) with 
max_active set to max_threads divided by some arbitrary constant (3? 4?)?

regards,
-- 
js
suse labs