On Thu, 17 Jul 2025 07:26:59 +0200 Jiri Slaby jirislaby@kernel.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@vger.kernel.org Signed-off-by: Hauke Mehrtens hauke@hauke-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,