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

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Message-ID: <20081106072352.GR21867@kernel.dk>
Date:	Thu, 6 Nov 2008 08:23:54 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	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, 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.

-- 
Jens Axboe

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