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Subject[PATCH BUGFIX IMPROVEMENT 06/14] block, bfq: split function bfq_better_to_idle
Date
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>
---
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;
+
+ /*
+ * 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 all the components we need to compute the
+ * 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;
}

/*
--
2.20.1
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