Re: [PATCH v3 2/2] modules/kmod: replace implementation with a semaphore

[David Hildenbrand <david@redhat.com>]

On 14.04.23 07:13, Luis Chamberlain wrote:
> Simplify the concurrency delimiter we use for kmod with the semaphore.
> I had used the kmod strategy to try to implement a similar concurrency
> delimiter for the kernel_read*() calls from the finit_module() path
> so to reduce vmalloc() memory pressure. That effort didn't provide yet
> conclusive results, but one thing that became clear is we can use
> the suggested alternative solution with semaphores which Linus hinted
> at instead of using the atomic / wait strategy.
> 
> I've stress tested this with kmod test 0008:
> 
> time /data/linux-next/tools/testing/selftests/kmod/kmod.sh -t 0008
> 
> And I get only a *slight* delay. That delay however is small, a few
> seconds for a full test loop run that runs 150 times, for about ~30-40
> seconds. The small delay is worth the simplfication IMHO.
> 
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
>   kernel/module/kmod.c | 26 +++++++-------------------
>   1 file changed, 7 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/module/kmod.c b/kernel/module/kmod.c
> index b717134ebe17..5899083436a3 100644
> --- a/kernel/module/kmod.c
> +++ b/kernel/module/kmod.c
> @@ -40,8 +40,7 @@
>    * effect. Systems like these are very unlikely if modules are enabled.
>    */
>   #define MAX_KMOD_CONCURRENT 50
> -static atomic_t kmod_concurrent_max = ATOMIC_INIT(MAX_KMOD_CONCURRENT);
> -static DECLARE_WAIT_QUEUE_HEAD(kmod_wq);
> +static DEFINE_SEMAPHORE(kmod_concurrent_max, MAX_KMOD_CONCURRENT);
>   
>   /*
>    * This is a restriction on having *all* MAX_KMOD_CONCURRENT threads
> @@ -148,29 +147,18 @@ int __request_module(bool wait, const char *fmt, ...)
>   	if (ret)
>   		return ret;
>   
> -	if (atomic_dec_if_positive(&kmod_concurrent_max) < 0) {
> -		pr_warn_ratelimited("request_module: kmod_concurrent_max (%u) close to 0 (max_modprobes: %u), for module %s, throttling...",
> -				    atomic_read(&kmod_concurrent_max),
> -				    MAX_KMOD_CONCURRENT, module_name);
> -		ret = wait_event_killable_timeout(kmod_wq,
> -						  atomic_dec_if_positive(&kmod_concurrent_max) >= 0,
> -						  MAX_KMOD_ALL_BUSY_TIMEOUT * HZ);
> -		if (!ret) {
> -			pr_warn_ratelimited("request_module: modprobe %s cannot be processed, kmod busy with %d threads for more than %d seconds now",
> -					    module_name, MAX_KMOD_CONCURRENT, MAX_KMOD_ALL_BUSY_TIMEOUT);
> -			return -ETIME;
> -		} else if (ret == -ERESTARTSYS) {
> -			pr_warn_ratelimited("request_module: sigkill sent for modprobe %s, giving up", module_name);
> -			return ret;
> -		}
> +	ret = down_timeout(&kmod_concurrent_max, MAX_KMOD_ALL_BUSY_TIMEOUT * HZ);
> +	if (ret) {
> +		pr_warn_ratelimited("request_module: modprobe %s cannot be processed, kmod busy with %d threads for more than %d seconds now",
> +				    module_name, MAX_KMOD_CONCURRENT, MAX_KMOD_ALL_BUSY_TIMEOUT);
> +		return ret;
>   	}
>   
>   	trace_module_request(module_name, wait, _RET_IP_);
>   
>   	ret = call_modprobe(module_name, wait ? UMH_WAIT_PROC : UMH_WAIT_EXEC);
>   
> -	atomic_inc(&kmod_concurrent_max);
> -	wake_up(&kmod_wq);
> +	up(&kmod_concurrent_max);
>   
>   	return ret;
>   }

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb