From 05c2f5c30b3ca2346a5bb7c74b0c9515d8f4fbd2 Mon Sep 17 00:00:00 2001 From: Paolo Valente Date: Tue, 29 Jan 2019 12:06:30 +0100 Subject: [PATCH] block, bfq: split function bfq_better_to_idle This is a preparatory commit for commits that need to check only one of the two main reasons for idling. This change should also improve the quality of the code a little bit, by splitting a function that contains very long, non-trivial and little related comments. Signed-off-by: Paolo Valente Signed-off-by: Jens Axboe --- block/bfq-iosched.c | 155 +++++++++++++++++++++++--------------------- 1 file changed, 82 insertions(+), 73 deletions(-) diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 6bfbfa65610b..2756f4b1432b 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -3404,53 +3404,13 @@ static bool bfq_may_expire_for_budg_timeout(struct bfq_queue *bfqq) bfq_bfqq_budget_timeout(bfqq); } -/* - * For a queue that becomes empty, device idling is allowed only if - * this function returns true for the queue. As a consequence, since - * device idling plays a critical role in both throughput boosting and - * service guarantees, the return value of this function plays a - * critical role in both these aspects as well. - * - * In a nutshell, this function returns true only if idling is - * beneficial for throughput or, even if detrimental for throughput, - * idling is however necessary to preserve service guarantees (low - * latency, desired throughput distribution, ...). In particular, on - * NCQ-capable devices, this function tries to return false, so as to - * help keep the drives' internal queues full, whenever this helps the - * device boost the throughput without causing any service-guarantee - * issue. - * - * In more detail, the return value of this function is obtained by, - * first, computing a number of boolean variables that take into - * account throughput and service-guarantee issues, and, then, - * combining these variables in a logical expression. Most of the - * issues taken into account are not trivial. We discuss these issues - * individually while introducing the variables. - */ -static bool bfq_better_to_idle(struct bfq_queue *bfqq) +static bool idling_boosts_thr_without_issues(struct bfq_data *bfqd, + struct bfq_queue *bfqq) { - struct bfq_data *bfqd = bfqq->bfqd; bool rot_without_queueing = !blk_queue_nonrot(bfqd->queue) && !bfqd->hw_tag, bfqq_sequential_and_IO_bound, - idling_boosts_thr, idling_boosts_thr_without_issues, - idling_needed_for_service_guarantees, - asymmetric_scenario; - - if (bfqd->strict_guarantees) - return true; - - /* - * Idling is performed only if slice_idle > 0. In addition, we - * do not idle if - * (a) bfqq is async - * (b) bfqq is in the idle io prio class: in this case we do - * not idle because we want to minimize the bandwidth that - * queues in this class can steal to higher-priority queues - */ - if (bfqd->bfq_slice_idle == 0 || !bfq_bfqq_sync(bfqq) || - bfq_class_idle(bfqq)) - return false; + idling_boosts_thr; bfqq_sequential_and_IO_bound = !BFQQ_SEEKY(bfqq) && bfq_bfqq_IO_bound(bfqq) && bfq_bfqq_has_short_ttime(bfqq); @@ -3482,8 +3442,7 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq) bfqq_sequential_and_IO_bound); /* - * The value of the next variable, - * idling_boosts_thr_without_issues, is equal to that of + * The return value of this function is equal to that of * idling_boosts_thr, unless a special case holds. In this * special case, described below, idling may cause problems to * weight-raised queues. @@ -3500,32 +3459,35 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq) * which enqueue several requests in advance, and further * reorder internally-queued requests. * - * For this reason, we force to false the value of - * idling_boosts_thr_without_issues if there are weight-raised - * busy queues. In this case, and if bfqq is not weight-raised, - * this guarantees that the device is not idled for bfqq (if, - * instead, bfqq is weight-raised, then idling will be - * guaranteed by another variable, see below). Combined with - * the timestamping rules of BFQ (see [1] for details), this - * behavior causes bfqq, and hence any sync non-weight-raised - * queue, to get a lower number of requests served, and thus - * to ask for a lower number of requests from the request - * pool, before the busy weight-raised queues get served - * again. This often mitigates starvation problems in the - * presence of heavy write workloads and NCQ, thereby - * guaranteeing a higher application and system responsiveness - * in these hostile scenarios. + * For this reason, we force to false the return value if + * there are weight-raised busy queues. In this case, and if + * bfqq is not weight-raised, this guarantees that the device + * is not idled for bfqq (if, instead, bfqq is weight-raised, + * then idling will be guaranteed by another variable, see + * below). Combined with the timestamping rules of BFQ (see + * [1] for details), this behavior causes bfqq, and hence any + * sync non-weight-raised queue, to get a lower number of + * requests served, and thus to ask for a lower number of + * requests from the request pool, before the busy + * weight-raised queues get served again. This often mitigates + * starvation problems in the presence of heavy write + * workloads and NCQ, thereby guaranteeing a higher + * application and system responsiveness in these hostile + * scenarios. */ - idling_boosts_thr_without_issues = idling_boosts_thr && + return idling_boosts_thr && bfqd->wr_busy_queues == 0; +} +static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd, + struct bfq_queue *bfqq) +{ /* - * There is then a case where idling must be performed not - * for throughput concerns, but to preserve service - * guarantees. + * There is a case where idling must be performed not for + * throughput concerns, but to preserve service guarantees. * * To introduce this case, we can note that allowing the drive - * to enqueue more than one request at a time, and hence + * to enqueue more than one request at a time, and thereby * delegating de facto final scheduling decisions to the * drive's internal scheduler, entails loss of control on the * actual request service order. In particular, the critical @@ -3682,9 +3644,9 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq) * to let requests be served in the desired order until all * the requests already queued in the device have been served. */ - asymmetric_scenario = (bfqq->wr_coeff > 1 && - bfqd->wr_busy_queues < - bfq_tot_busy_queues(bfqd)) || + bool asymmetric_scenario = (bfqq->wr_coeff > 1 && + bfqd->wr_busy_queues < + bfq_tot_busy_queues(bfqd)) || !bfq_symmetric_scenario(bfqd); /* @@ -3701,17 +3663,64 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq) * now establish when idling is actually needed to preserve * service guarantees. */ - idling_needed_for_service_guarantees = - asymmetric_scenario && !bfq_bfqq_in_large_burst(bfqq); + return asymmetric_scenario && !bfq_bfqq_in_large_burst(bfqq); +} + +/* + * For a queue that becomes empty, device idling is allowed only if + * this function returns true for that queue. As a consequence, since + * device idling plays a critical role for both throughput boosting + * and service guarantees, the return value of this function plays a + * critical role as well. + * + * In a nutshell, this function returns true only if idling is + * beneficial for throughput or, even if detrimental for throughput, + * idling is however necessary to preserve service guarantees (low + * latency, desired throughput distribution, ...). In particular, on + * NCQ-capable devices, this function tries to return false, so as to + * help keep the drives' internal queues full, whenever this helps the + * device boost the throughput without causing any service-guarantee + * issue. + * + * Most of the issues taken into account to get the return value of + * this function are not trivial. We discuss these issues in the two + * functions providing the main pieces of information needed by this + * function. + */ +static bool bfq_better_to_idle(struct bfq_queue *bfqq) +{ + struct bfq_data *bfqd = bfqq->bfqd; + bool idling_boosts_thr_with_no_issue, idling_needed_for_service_guar; + + if (unlikely(bfqd->strict_guarantees)) + return true; /* - * We have now all the components we need to compute the + * Idling is performed only if slice_idle > 0. In addition, we + * do not idle if + * (a) bfqq is async + * (b) bfqq is in the idle io prio class: in this case we do + * not idle because we want to minimize the bandwidth that + * queues in this class can steal to higher-priority queues + */ + if (bfqd->bfq_slice_idle == 0 || !bfq_bfqq_sync(bfqq) || + bfq_class_idle(bfqq)) + return false; + + idling_boosts_thr_with_no_issue = + idling_boosts_thr_without_issues(bfqd, bfqq); + + idling_needed_for_service_guar = + idling_needed_for_service_guarantees(bfqd, bfqq); + + /* + * We have now the two components we need to compute the * return value of the function, which is true only if idling * either boosts the throughput (without issues), or is * necessary to preserve service guarantees. */ - return idling_boosts_thr_without_issues || - idling_needed_for_service_guarantees; + return idling_boosts_thr_with_no_issue || + idling_needed_for_service_guar; } /*