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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 <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente
Jens Axboe
parent
73d5811849
commit
05c2f5c30b
@ -3404,53 +3404,13 @@ static bool bfq_may_expire_for_budg_timeout(struct bfq_queue *bfqq)
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bfq_bfqq_budget_timeout(bfqq);
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}
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/*
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* For a queue that becomes empty, device idling is allowed only if
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* this function returns true for the queue. As a consequence, since
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* device idling plays a critical role in both throughput boosting and
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* service guarantees, the return value of this function plays a
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* critical role in both these aspects as well.
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*
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* In a nutshell, this function returns true only if idling is
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* beneficial for throughput or, even if detrimental for throughput,
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* idling is however necessary to preserve service guarantees (low
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* latency, desired throughput distribution, ...). In particular, on
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* NCQ-capable devices, this function tries to return false, so as to
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* help keep the drives' internal queues full, whenever this helps the
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* device boost the throughput without causing any service-guarantee
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* issue.
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*
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* In more detail, the return value of this function is obtained by,
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* first, computing a number of boolean variables that take into
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* account throughput and service-guarantee issues, and, then,
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* combining these variables in a logical expression. Most of the
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* issues taken into account are not trivial. We discuss these issues
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* individually while introducing the variables.
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*/
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static bool bfq_better_to_idle(struct bfq_queue *bfqq)
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static bool idling_boosts_thr_without_issues(struct bfq_data *bfqd,
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struct bfq_queue *bfqq)
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{
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struct bfq_data *bfqd = bfqq->bfqd;
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bool rot_without_queueing =
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!blk_queue_nonrot(bfqd->queue) && !bfqd->hw_tag,
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bfqq_sequential_and_IO_bound,
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idling_boosts_thr, idling_boosts_thr_without_issues,
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idling_needed_for_service_guarantees,
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asymmetric_scenario;
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if (bfqd->strict_guarantees)
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return true;
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/*
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* Idling is performed only if slice_idle > 0. In addition, we
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* do not idle if
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* (a) bfqq is async
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* (b) bfqq is in the idle io prio class: in this case we do
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* not idle because we want to minimize the bandwidth that
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* queues in this class can steal to higher-priority queues
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*/
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if (bfqd->bfq_slice_idle == 0 || !bfq_bfqq_sync(bfqq) ||
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bfq_class_idle(bfqq))
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return false;
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idling_boosts_thr;
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bfqq_sequential_and_IO_bound = !BFQQ_SEEKY(bfqq) &&
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bfq_bfqq_IO_bound(bfqq) && bfq_bfqq_has_short_ttime(bfqq);
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@ -3482,8 +3442,7 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq)
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bfqq_sequential_and_IO_bound);
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/*
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* The value of the next variable,
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* idling_boosts_thr_without_issues, is equal to that of
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* The return value of this function is equal to that of
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* idling_boosts_thr, unless a special case holds. In this
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* special case, described below, idling may cause problems to
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* weight-raised queues.
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@ -3500,32 +3459,35 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq)
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* which enqueue several requests in advance, and further
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* reorder internally-queued requests.
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*
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* For this reason, we force to false the value of
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* idling_boosts_thr_without_issues if there are weight-raised
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* busy queues. In this case, and if bfqq is not weight-raised,
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* this guarantees that the device is not idled for bfqq (if,
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* instead, bfqq is weight-raised, then idling will be
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* guaranteed by another variable, see below). Combined with
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* the timestamping rules of BFQ (see [1] for details), this
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* behavior causes bfqq, and hence any sync non-weight-raised
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* queue, to get a lower number of requests served, and thus
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* to ask for a lower number of requests from the request
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* pool, before the busy weight-raised queues get served
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* again. This often mitigates starvation problems in the
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* presence of heavy write workloads and NCQ, thereby
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* guaranteeing a higher application and system responsiveness
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* in these hostile scenarios.
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* For this reason, we force to false the return value if
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* there are weight-raised busy queues. In this case, and if
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* bfqq is not weight-raised, this guarantees that the device
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* is not idled for bfqq (if, instead, bfqq is weight-raised,
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* then idling will be guaranteed by another variable, see
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* below). Combined with the timestamping rules of BFQ (see
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* [1] for details), this behavior causes bfqq, and hence any
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* sync non-weight-raised queue, to get a lower number of
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* requests served, and thus to ask for a lower number of
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* requests from the request pool, before the busy
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* weight-raised queues get served again. This often mitigates
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* starvation problems in the presence of heavy write
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* workloads and NCQ, thereby guaranteeing a higher
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* application and system responsiveness in these hostile
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* scenarios.
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*/
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idling_boosts_thr_without_issues = idling_boosts_thr &&
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return idling_boosts_thr &&
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bfqd->wr_busy_queues == 0;
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}
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static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
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struct bfq_queue *bfqq)
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{
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/*
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* There is then a case where idling must be performed not
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* for throughput concerns, but to preserve service
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* guarantees.
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* There is a case where idling must be performed not for
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* throughput concerns, but to preserve service guarantees.
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*
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* To introduce this case, we can note that allowing the drive
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* to enqueue more than one request at a time, and hence
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* to enqueue more than one request at a time, and thereby
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* delegating de facto final scheduling decisions to the
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* drive's internal scheduler, entails loss of control on the
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* actual request service order. In particular, the critical
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@ -3682,9 +3644,9 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq)
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* to let requests be served in the desired order until all
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* the requests already queued in the device have been served.
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*/
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asymmetric_scenario = (bfqq->wr_coeff > 1 &&
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bfqd->wr_busy_queues <
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bfq_tot_busy_queues(bfqd)) ||
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bool asymmetric_scenario = (bfqq->wr_coeff > 1 &&
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bfqd->wr_busy_queues <
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bfq_tot_busy_queues(bfqd)) ||
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!bfq_symmetric_scenario(bfqd);
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/*
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@ -3701,17 +3663,64 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq)
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* now establish when idling is actually needed to preserve
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* service guarantees.
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*/
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idling_needed_for_service_guarantees =
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asymmetric_scenario && !bfq_bfqq_in_large_burst(bfqq);
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return asymmetric_scenario && !bfq_bfqq_in_large_burst(bfqq);
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}
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/*
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* For a queue that becomes empty, device idling is allowed only if
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* this function returns true for that queue. As a consequence, since
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* device idling plays a critical role for both throughput boosting
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* and service guarantees, the return value of this function plays a
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* critical role as well.
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*
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* In a nutshell, this function returns true only if idling is
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* beneficial for throughput or, even if detrimental for throughput,
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* idling is however necessary to preserve service guarantees (low
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* latency, desired throughput distribution, ...). In particular, on
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* NCQ-capable devices, this function tries to return false, so as to
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* help keep the drives' internal queues full, whenever this helps the
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* device boost the throughput without causing any service-guarantee
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* issue.
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*
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* Most of the issues taken into account to get the return value of
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* this function are not trivial. We discuss these issues in the two
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* functions providing the main pieces of information needed by this
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* function.
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*/
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static bool bfq_better_to_idle(struct bfq_queue *bfqq)
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{
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struct bfq_data *bfqd = bfqq->bfqd;
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bool idling_boosts_thr_with_no_issue, idling_needed_for_service_guar;
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if (unlikely(bfqd->strict_guarantees))
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return true;
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/*
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* We have now all the components we need to compute the
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* Idling is performed only if slice_idle > 0. In addition, we
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* do not idle if
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* (a) bfqq is async
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* (b) bfqq is in the idle io prio class: in this case we do
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* not idle because we want to minimize the bandwidth that
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* queues in this class can steal to higher-priority queues
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*/
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if (bfqd->bfq_slice_idle == 0 || !bfq_bfqq_sync(bfqq) ||
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bfq_class_idle(bfqq))
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return false;
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idling_boosts_thr_with_no_issue =
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idling_boosts_thr_without_issues(bfqd, bfqq);
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idling_needed_for_service_guar =
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idling_needed_for_service_guarantees(bfqd, bfqq);
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/*
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* We have now the two components we need to compute the
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* return value of the function, which is true only if idling
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* either boosts the throughput (without issues), or is
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* necessary to preserve service guarantees.
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*/
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return idling_boosts_thr_without_issues ||
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idling_needed_for_service_guarantees;
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return idling_boosts_thr_with_no_issue ||
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idling_needed_for_service_guar;
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}
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/*
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