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

[Breno Leitao]

hello Jens,

On Fri, Oct 11, 2024 at 10:02:13AM -0600, Jens Axboe wrote:
> 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.

Sure, let me send a v3 without the helper.

Thanks for the review
--breno