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Date:   Thu, 13 Apr 2023 11:44:51 +0200 (CEST)
From:   Miroslav Benes <mbenes@suse.cz>
To:     Luis Chamberlain <mcgrof@kernel.org>
cc:     david@redhat.com, patches@lists.linux.dev,
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Subject: Re: [PATCH v2 2/2] modules/kmod: replace implementation with a
 sempahore
In-Reply-To: <20230405203505.1343562-3-mcgrof@kernel.org>
Message-ID: <alpine.LSU.2.21.2304131136580.27633@pobox.suse.cz>
References: <20230405203505.1343562-1-mcgrof@kernel.org> <20230405203505.1343562-3-mcgrof@kernel.org>
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Hi,

On Wed, 5 Apr 2023, Luis Chamberlain wrote:

s/sempahore/semaphore/ in the subject

> Simplfy the concurrency delimiter we user for kmod with the semaphore.

"Simplify the concurrency delimiter we use for kmod with the semaphore."

(two typos)

> 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 provid yet

s/provid/provide/

> conclusive results, but one thing that did became clear is we can use

s/did // (or s/became/become/)

> 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..925eb85b8346 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);

MAX_KMOD_ALL_BUSY_TIMEOUT * HZ ?

The simplification is very nice.

Miroslav