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

[Jiri Slaby]

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@xxxxxxxxxxxxxxx
> Signed-off-by: Hauke Mehrtens <hauke@xxxxxxxxxx>
> ---
>   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