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

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Message-Id: <20081107130552K.fujita.tomonori@lab.ntt.co.jp>
Date:	Fri, 7 Nov 2008 13:05:55 +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 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).
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