From 3a9a3f6cc55418dd1525e636dccbbe13c394f652 Mon Sep 17 00:00:00 2001 From: Divyesh Shah Date: Fri, 30 Jan 2009 12:46:41 +0100 Subject: [PATCH] cfq-iosched: Allow RT requests to pre-empt ongoing BE timeslice 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. 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 Signed-off-by: Divyesh Shah Signed-off-by: Jens Axboe --- block/cfq-iosched.c | 39 ++++++++++++++++++++++++++++++++++++++- 1 file changed, 38 insertions(+), 1 deletion(-) diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index e8525fa..664ebfd 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); -- 2.7.4