linux-kernel - Re: Problems with the block-layer timeouts

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Date:	Tue, 11 Nov 2008 15:54:29 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	jens.axboe@...cle.com
Cc:	fujita.tomonori@....ntt.co.jp, stern@...land.harvard.edu,
	James.Bottomley@...senPartnership.com, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Problems with the block-layer timeouts

On Fri, 7 Nov 2008 12:24:50 +0100
Jens Axboe <jens.axboe@...cle.com> wrote:

> On Fri, Nov 07 2008, FUJITA Tomonori wrote:
> > On Thu, 6 Nov 2008 08:23:54 +0100
> > Jens Axboe <jens.axboe@...cle.com> wrote:
> > 
> > > On Thu, Nov 06 2008, FUJITA Tomonori wrote:
> > > > On Mon, 3 Nov 2008 09:52:48 +0100
> > > > Jens Axboe <jens.axboe@...cle.com> wrote:
> > > > 
> > > > > > In blk_del_timer(), there's no reason to test q->rq_timed_out_fn.  If 
> > > > > > the method pointer is NULL then req->deadline would be 0 anyway.  In 
> > > > > > addition, req->deadline should be set to 0 and the end of the routine, 
> > > > > > just in case req gets requeued.
> > > > > > 
> > > > > > In blk_add_timer(), the line
> > > > > > 
> > > > > > 	expiry = round_jiffies(req->deadline);
> > > > > > 
> > > > > > is not optimal.  round_jiffies() will sometimes round a value _down_ to
> > > > > > the nearest second.  But blk_rq_timed_out_timer() tests whether
> > > > > > req->deadline is in the past -- and if the deadline was rounded down
> > > > > > then this won't be true the first time through.  You wind up getting an
> > > > > > unnecessary timer interrupt.  Instead there should be a
> > > > > > round_jiffies_up() utility routine, and it should be used in both
> > > > > > blk_add_timer() and blk_rq_timed_out_timer().
> > > > > 
> > > > > Very good point, we do indeed want a round_jiffies_up() for this!
> > > > 
> > > > Just out of curiosity, why do we need to use round_jiffies here? We
> > > > didn't do that for SCSI, right?
> > > 
> > > We don't have to, but given that we don't care about exact timeouts, we
> > > may as well. It's not a new thing, we've done that since pretty much the
> > > beginning of the generic timeout development.
> > 
> > I'm not sure that the users of the timeout feature can control exact
> > timeouts because the block layer doesn't let the users directly play
> > with the timer. elv_dequeue_request() is not the exact time that the
> > users want to start the timer. Instead, the block layer hides the
> > details behind the elevator (note that as I said before, I think that
> > it's the right thing). So the round_jiffies in the block layer doesn't
> > make sense to me. I prefer remove them instead of adding a bunch of
> > round_jiffies_up_* (I bet that some of them will never be used).
> 
> I don't understand your concern, to be honest. We only need to round up
> once, and that is when we add/mod the timer. And we do that simply to
> play nice and group the timout with other timers, to save a bit of
> power.

I don't worry about anything. I just think that these round_jiffies_up
are pointless because they were added for the block-layer users that
care about exact timeouts, however the block-layer doesn't export
blk_add_timer() so the block-layer users can't control the exact time
when the timer starts. So doing round_jiffies_up calculation per every
request doesn't make sense for me.

Anyway, it's trivial. If you like these round_jiffies_up, it's fine by
me.
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