Re: [PATCH v2] elevator: do not request_module if elevator exists

[Jens Axboe]

On 10/11/24 9:49 AM, 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")
> Fixes: 734e1a860312 ("block: Prevent deadlocks when switching elevators")
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> ---
> Changelog:
>  v2:
>    * Protecet __elevator_find() by elv_list_lock (Christoph Hellwig)
>  v1:
>    * https://lore.kernel.org/all/20241010141509.4028059-1-leitao@xxxxxxxxxx/
> 
>  block/elevator.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/block/elevator.c b/block/elevator.c
> index 565807f0b1c7..1ac9be3e47d1 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -106,6 +106,17 @@ static struct elevator_type *__elevator_find(const char *name)
>  	return NULL;
>  }
>  
> +static struct elevator_type *elevator_find(const char *name)
> +{
> +	struct elevator_type *e;
> +
> +	spin_lock(&elv_list_lock);
> +	e = __elevator_find(name);
> +	spin_unlock(&elv_list_lock);
> +
> +	return e;
> +}

Probably just drop this helper. If it's only used in one spot, then we
don't need to add a helper for it.

-- 
Jens Axboe