linux-kernel - Re: [PATCH] elevator: do not request

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Message-ID: <20241011-spotted-cormorant-of-research-0d8184@leitao>
Date: Fri, 11 Oct 2024 05:37:47 -0700
From: Breno Leitao <leitao@...ian.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: Jens Axboe <axboe@...nel.dk>, kernel-team@...a.com,
	"open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] elevator: do not request_module if elevator exists

Hello Christoph,

On Fri, Oct 11, 2024 at 01:24:15AM -0700, Christoph Hellwig wrote:
> 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@...ian.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.

That is right. Thanks for the heads-up.

I will send a fix soon.