linux-kernel - Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler

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Date:	Tue, 19 Feb 2008 10:24:28 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Paul Clements <paul.clements@...eleye.com>,
	randy.dunlap@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler

On Tue, Feb 19 2008, Jens Axboe wrote:
> On Mon, Feb 18 2008, Andrew Morton wrote:
> > On Mon, 18 Feb 2008 19:16:30 +0100 Jens Axboe <jens.axboe@...cle.com> wrote:
> > 
> > > On Tue, Feb 12 2008, Andrew Morton wrote:
> > > > On Sat, 09 Feb 2008 08:30:40 -0500
> > > > Paul Clements <paul.clements@...eleye.com> wrote:
> > > > 
> > > > > +		old_e = disk->queue->elevator;
> > > > > +		if (elevator_init(disk->queue, "deadline") == 0 ||
> > > > > +			elevator_init(disk->queue, "noop") == 0) {
> > > > > +				elevator_exit(old_e);
> > > > > +		}
> > > > >  	}
> > > > 
> > > > afacit elevator_init() will not trigger a request_module().  And you really
> > > > do want to trigger the request_module() here.  Perhaps the block layer
> > > > should provide a means of doing so?
> > > 
> > > Good point, I think elevator_get() should do that automatically. Does
> > > this look sane?
> > > 
> > > diff --git a/block/elevator.c b/block/elevator.c
> > > index bafbae0..88318c3 100644
> > > --- a/block/elevator.c
> > > +++ b/block/elevator.c
> > > @@ -134,6 +134,21 @@ static struct elevator_type *elevator_get(const char *name)
> > >  	spin_lock(&elv_list_lock);
> > >  
> > >  	e = elevator_find(name);
> > > +	if (!e) {
> > > +		char elv[ELV_NAME_MAX + strlen("-iosched")];
> > > +
> > > +		spin_unlock(&elv_list_lock);
> > > +
> > > +		if (!strcmp(name, "anticipatory"))
> > > +			sprintf(elv, "as-iosched");
> > > +		else
> > > +			sprintf(elv, "%s-iosched", name);
> > > +
> > > +		request_module(elv);
> > > +		spin_lock(&elv_list_lock);
> > > +		e = elevator_find(name);
> > > +	}
> > > +
> > >  	if (e && !try_module_get(e->elevator_owner))
> > >  		e = NULL;
> > 
> > Looks nice and simple.  There might be some of the usual ordering problems
> > when this is called during boot, maybe is-initramfs-available-yet problems,
> > etc.  But it's unlikely to make things regress from where they are now.
> 
> Isn't request_module() and below robust enough to handle that?

BTW, I've verified that it works as expected (at least after boot):

carl:/sys/block/sda/queue # cat scheduler 
noop [cfq] 
carl:/sys/block/sda/queue # echo anticipatory > scheduler 
carl:/sys/block/sda/queue # dmesg
[...]
io scheduler anticipatory registered
carl:/sys/block/sda/queue # cat scheduler 
noop cfq [anticipatory] 

So it properly loads as-iosched instead of failing, like it would have
done before and required the user to do a modprobe as-iosched first.

carl:/sys/block/sda/queue # echo foobar > scheduler 
-bash: echo: write error: Invalid argument
carl:/sys/block/sda/queue # dmesg
[...]
elevator: type foobar not found

-- 
Jens Axboe

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