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

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Message-ID: <20080219091943.GS23197@kernel.dk>
Date:	Tue, 19 Feb 2008 10:19:43 +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 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?

> Should we emit a warning if the desired elevator wasn't available?

Hmm, not sure. Either the request came from a driver, in which case
it'll be notified that we could not load that elevator. Or it'll come
through sysfs online switching, in which case we already print a
warning.

-- 
Jens Axboe

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