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    SubjectRe: Problems with the block-layer timeouts
    On Fri, Nov 07 2008, FUJITA Tomonori wrote:
    > On Thu, 6 Nov 2008 08:23:54 +0100
    > Jens Axboe <jens.axboe@oracle.com> wrote:
    >
    > > On Thu, Nov 06 2008, FUJITA Tomonori wrote:
    > > > On Mon, 3 Nov 2008 09:52:48 +0100
    > > > Jens Axboe <jens.axboe@oracle.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.

    --
    Jens Axboe



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