Re: [PATCH] elevator: do not request_module if elevator exists

[Christoph Hellwig <hch@infradead.org>]

On Thu, Oct 10, 2024 at 07:15:08AM -0700, Breno Leitao wrote:
> Whenever an I/O elevator is changed, the system attempts to load a
> module for the new elevator. This occurs regardless of whether the
> elevator is already loaded or built directly into the kernel. This
> behavior introduces unnecessary overhead and potential issues.
> 
> This makes the operation slower, and more error-prone. For instance,
> making the problem fixed by [1] visible for users that doesn't even rely
> on modules being available through modules.
> 
> Do not try to load the ioscheduler if it is already visible.
> 
> This change brings two main benefits: it improves the performance of
> elevator changes, and it reduces the likelihood of errors occurring
> during this process.
> 
> [1] Commit e3accac1a976 ("block: Fix elv_iosched_local_module handling of "none" scheduler")
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  block/elevator.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/block/elevator.c b/block/elevator.c
> index 4122026b11f1..1904e217505a 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -709,13 +709,16 @@ int elv_iosched_load_module(struct gendisk *disk, const char *buf,
>  			    size_t count)
>  {
>  	char elevator_name[ELV_NAME_MAX];
> +	const char *name;
>  
>  	if (!elv_support_iosched(disk->queue))
>  		return -EOPNOTSUPP;
>  
>  	strscpy(elevator_name, buf, sizeof(elevator_name));
> +	name = strstrip(elevator_name);
>  
> -	request_module("%s-iosched", strstrip(elevator_name));
> +	if (!__elevator_find(name))

__elevator_find needs to be called with elv_list_lock.