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Message-ID: <20090130114941.GN30821@kernel.dk>
Date: Fri, 30 Jan 2009 12:49:41 +0100
From: Jens Axboe <jens.axboe@...cle.com>
To: Divyesh Shah <dpshah@...gle.com>
Cc: mikew@...gle.com, nauman@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2][RESEND] Allow RT requests to pre-empt ongoing BE timeslice in CFQ.
On Fri, Jan 30 2009, Jens Axboe wrote:
> On Wed, Jan 28 2009, Divyesh Shah wrote:
>
> Strange, I was pretty sure this was added, but apparently not. Can you
> resend a non-whitespace damaged version and I'll make sure to add it to
> the mix.
Nevermind, found non-cc'ed original on lkml, which wasn't whitespace
damaged.
>
>
> > + Jens Axboe
> >
> > On Wed, Jan 28, 2009 at 11:35 AM, Divyesh Shah <dpshah@...gle.com> wrote:
> > > This patch adds the ability to pre-empt an ongoing BE timeslice when a RT
> > > request is waiting for the current timeslice to complete. This reduces the
> > > wait time to disk for RT requests from an upper bound of 4 (current value
> > > of cfq_quantum) to 1 disk request.
> > >
> > > Signed-off-by: Divyesh Shah <dpshah@...gle.com>
> > > ---
> > > Applied Jens' suggeested changes to avoid the rb lookup and use !cfq_class_rt()
> > > and retested.
> > >
> > > Latency(secs) for the RT task when doing sequential reads from 10G file.
> > > | only RT | RT + BE | RT + BE + this patch
> > > small (512 byte) reads | 143 | 163 | 145
> > > large (1Mb) reads | 142 | 158 | 146
> > >
> > > block/cfq-iosched.c | 39 ++++++++++++++++++++++++++++++++++++++-
> > > 1 files changed, 38 insertions(+), 1 deletions(-)
> > >
> > > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > > index e8525fa..5fec9e5 100644
> > > --- a/block/cfq-iosched.c
> > > +++ b/block/cfq-iosched.c
> > > @@ -84,6 +84,11 @@ struct cfq_data {
> > > */
> > > struct cfq_rb_root service_tree;
> > > unsigned int busy_queues;
> > > + /*
> > > + * Used to track any pending rt requests so we can pre-empt current
> > > + * non-RT cfqq in service when this value is non-zero.
> > > + */
> > > + unsigned int busy_rt_queues;
> > >
> > > int rq_in_driver;
> > > int sync_flight;
> > > @@ -562,6 +567,8 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
> > > BUG_ON(cfq_cfqq_on_rr(cfqq));
> > > cfq_mark_cfqq_on_rr(cfqq);
> > > cfqd->busy_queues++;
> > > + if (cfq_class_rt(cfqq))
> > > + cfqd->busy_rt_queues++;
> > >
> > > cfq_resort_rr_list(cfqd, cfqq);
> > > }
> > > @@ -581,6 +588,8 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
> > >
> > > BUG_ON(!cfqd->busy_queues);
> > > cfqd->busy_queues--;
> > > + if (cfq_class_rt(cfqq))
> > > + cfqd->busy_rt_queues--;
> > > }
> > >
> > > /*
> > > @@ -1005,6 +1014,20 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
> > > goto expire;
> > >
> > > /*
> > > + * If we have a RT cfqq waiting, then we pre-empt the current non-rt
> > > + * cfqq.
> > > + */
> > > + if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues) {
> > > + /*
> > > + * We simulate this as cfqq timed out so that it gets to bank
> > > + * the remaining of its time slice.
> > > + */
> > > + cfq_log_cfqq(cfqd, cfqq, "preempt");
> > > + cfq_slice_expired(cfqd, 1);
> > > + goto new_queue;
> > > + }
> > > +
> > > + /*
> > > * The active queue has requests and isn't expired, allow it to
> > > * dispatch.
> > > */
> > > @@ -1067,6 +1090,13 @@ __cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> > > if (RB_EMPTY_ROOT(&cfqq->sort_list))
> > > break;
> > >
> > > + /*
> > > + * If there is a non-empty RT cfqq waiting for current
> > > + * cfqq's timeslice to complete, pre-empt this cfqq
> > > + */
> > > + if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues)
> > > + break;
> > > +
> > > } while (dispatched < max_dispatch);
> > >
> > > /*
> > > @@ -1801,6 +1831,12 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
> > > if (rq_is_meta(rq) && !cfqq->meta_pending)
> > > return 1;
> > >
> > > + /*
> > > + * Allow an RT request to pre-empt an ongoing non-RT cfqq timeslice.
> > > + */
> > > + if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
> > > + return 1;
> > > +
> > > if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
> > > return 0;
> > >
> > > @@ -1870,7 +1906,8 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> > > /*
> > > * not the active queue - expire current slice if it is
> > > * idle and has expired it's mean thinktime or this new queue
> > > - * has some old slice time left and is of higher priority
> > > + * has some old slice time left and is of higher priority or
> > > + * this new queue is RT and the current one is BE
> > > */
> > > cfq_preempt_queue(cfqd, cfqq);
> > > cfq_mark_cfqq_must_dispatch(cfqq);
> > >
> > >
>
> --
> Jens Axboe
>
> --
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--
Jens Axboe
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