+cc: lkml
On Tue, Jan 6, 2009 at 5:38 PM, Divyesh Shah <[email protected]> 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 <[email protected]>
> ---
> 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 6a062ee..8afa2cf 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);
>
> /*
> @@ -1797,6 +1827,12 @@ 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;
>
> @@ -1866,7 +1902,8 @@ 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);
>
>