Messages in this thread Patch in this message | | | Date | Tue, 6 Jan 2009 09:59:41 +0100 | From | Jens Axboe <> | Subject | Re: [PATCH] Allow RT requests to pre-empt ongoing BE timeslice in CFQ. |
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On Mon, Jan 05 2009, Divyesh Shah 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@google.com> > --- > Patch based on 2.6.28 linus tree. > > Test Results: > 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 | 146 > large (1Mb) reads | 142 | 158 | 146
Results look good. I'm not too keen on doing the rb lookup, how about we just track pending rt queues instead? Add a counter to cfqd (busy_rq_queues) and inc/dec that along with busy_queues, then check that in the two locations? If that value is non-zero, an rt queue should be returned with cfq_rb_first() as well.
Also, the places that test cfq_class_be() seem more appropriate as !cfq_class_rt() checks, as that also covers idle queues.
How does the below look to you? It's untested here, but it compiles. I'd appreciate your comments and testing!
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index e8525fa..8640199 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -84,6 +84,7 @@ struct cfq_data { */ struct cfq_rb_root service_tree; unsigned int busy_queues; + unsigned int busy_rt_queues; int rq_in_driver; int sync_flight; @@ -562,6 +563,8 @@ 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 +584,8 @@ 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 +1010,20 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd) goto expire; /* + * If we have RT queues waiting, then we preempt 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 from RT"); + cfq_slice_expired(cfqd, 1); + goto new_queue; + } + + /* * The active queue has requests and isn't expired, allow it to * dispatch. */ @@ -1067,6 +1086,12 @@ __cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq, if (RB_EMPTY_ROOT(&cfqq->sort_list)) break; + /* + * If we have pending rt queues, stop dispatching + */ + if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues) + break; + } while (dispatched < max_dispatch); /* @@ -1801,6 +1826,12 @@ 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 BE 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; -- Jens Axboe
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