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

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Message-ID: <20250717223432.2a74e870@pumpkin>
Date: Thu, 17 Jul 2025 22:34:32 +0100
From: David Laight <david.laight.linux@...il.com>
To: Jiri Slaby <jirislaby@...nel.org>
Cc: Hauke Mehrtens <hauke@...ke-m.de>, sashal@...nel.org,
 linux-kernel@...r.kernel.org, frederic@...nel.org, david@...hat.com,
 viro@...iv.linux.org.uk, paulmck@...nel.org, stable@...r.kernel.org, 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

On Thu, 17 Jul 2025 07:26:59 +0200
Jiri Slaby <jirislaby@...nel.org> wrote:

> 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?)?

Or maybe just 1 ?
I'd guess all the threads either block in the same place or just block
each other??

	David

> 
> regards,