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

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Message-ID: <20250716124607.50fc5e34@pumpkin>
Date: Wed, 16 Jul 2025 12:46:07 +0100
From: David Laight <david.laight.linux@...il.com>
To: Hauke Mehrtens <hauke@...ke-m.de>
Cc: sashal@...nel.org, linux-kernel@...r.kernel.org, frederic@...nel.org,
 david@...hat.com, viro@...iv.linux.org.uk, paulmck@...nel.org, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org
Subject: Re: [PATCH] kernel/fork: Increase minimum number of allowed threads

On Sat, 12 Jul 2025 01:06:07 +0200
Hauke Mehrtens <hauke@...ke-m.de> wrote:

> On 7/12/25 01:03, Hauke Mehrtens wrote:
> > From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>  
> 
> Sorry this has the wrong from tag, will send a new patch.
> 
> Hauke>
> > 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 calles 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 bet it doesn't - it is likely to fail somewhere else instead.
While 20 is probably too low, the real issue seems to be that
the hotplug notifications need rate limiting.

	David

> > 
> > 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
> >   
> >   /*
> >    * Maximum number of threads  
> 
>