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Subject[PATCH V4 09/16] block, bfq: reduce latency during request-pool saturation
Date
This patch introduces an heuristic that reduces latency when the
I/O-request pool is saturated. This goal is achieved by disabling
device idling, for non-weight-raised queues, when there are weight-
raised queues with pending or in-flight requests. In fact, as
explained in more detail in the comment on the function
bfq_bfqq_may_idle(), this reduces the rate at which processes
associated with non-weight-raised queues grab requests from the pool,
thereby increasing the probability that processes associated with
weight-raised queues get a request immediately (or at least soon) when
they need one. Along the same line, if there are weight-raised queues,
then this patch halves the service rate of async (write) requests for
non-weight-raised queues.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
---
block/bfq-iosched.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 63 insertions(+), 3 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 574a5f6..deb1f21c 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -420,6 +420,8 @@ struct bfq_data {
* queue in service, even if it is idling).
*/
int busy_queues;
+ /* number of weight-raised busy @bfq_queues */
+ int wr_busy_queues;
/* number of queued requests */
int queued;
/* number of requests dispatched and waiting for completion */
@@ -2490,6 +2492,9 @@ static void bfq_del_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq,

bfqd->busy_queues--;

+ if (bfqq->wr_coeff > 1)
+ bfqd->wr_busy_queues--;
+
bfqg_stats_update_dequeue(bfqq_group(bfqq));

bfq_deactivate_bfqq(bfqd, bfqq, true, expiration);
@@ -2506,6 +2511,9 @@ static void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq)

bfq_mark_bfqq_busy(bfqq);
bfqd->busy_queues++;
+
+ if (bfqq->wr_coeff > 1)
+ bfqd->wr_busy_queues++;
}

#ifdef CONFIG_BFQ_GROUP_IOSCHED
@@ -3779,7 +3787,16 @@ static unsigned long bfq_serv_to_charge(struct request *rq,
if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1)
return blk_rq_sectors(rq);

- return blk_rq_sectors(rq) * bfq_async_charge_factor;
+ /*
+ * If there are no weight-raised queues, then amplify service
+ * by just the async charge factor; otherwise amplify service
+ * by twice the async charge factor, to further reduce latency
+ * for weight-raised queues.
+ */
+ if (bfqq->bfqd->wr_busy_queues == 0)
+ return blk_rq_sectors(rq) * bfq_async_charge_factor;
+
+ return blk_rq_sectors(rq) * 2 * bfq_async_charge_factor;
}

/**
@@ -4234,6 +4251,7 @@ static void bfq_add_request(struct request *rq)
bfqq->wr_coeff = bfqd->bfq_wr_coeff;
bfqq->wr_cur_max_time = bfq_wr_duration(bfqd);

+ bfqd->wr_busy_queues++;
bfqq->entity.prio_changed = 1;
}
if (prev != bfqq->next_rq)
@@ -4474,6 +4492,8 @@ static void bfq_requests_merged(struct request_queue *q, struct request *rq,
/* Must be called with bfqq != NULL */
static void bfq_bfqq_end_wr(struct bfq_queue *bfqq)
{
+ if (bfq_bfqq_busy(bfqq))
+ bfqq->bfqd->wr_busy_queues--;
bfqq->wr_coeff = 1;
bfqq->wr_cur_max_time = 0;
bfqq->last_wr_start_finish = jiffies;
@@ -5497,7 +5517,8 @@ static bool bfq_may_expire_for_budg_timeout(struct bfq_queue *bfqq)
static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
{
struct bfq_data *bfqd = bfqq->bfqd;
- bool idling_boosts_thr, asymmetric_scenario;
+ bool idling_boosts_thr, idling_boosts_thr_without_issues,
+ asymmetric_scenario;

if (bfqd->strict_guarantees)
return true;
@@ -5520,6 +5541,44 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
idling_boosts_thr = !bfqd->hw_tag || bfq_bfqq_IO_bound(bfqq);

/*
+ * The value of the next variable,
+ * idling_boosts_thr_without_issues, 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.
+ *
+ * When the request pool is saturated (e.g., in the presence
+ * of write hogs), if the processes associated with
+ * non-weight-raised queues ask for requests at a lower rate,
+ * then processes associated with weight-raised queues have a
+ * higher probability to get a request from the pool
+ * immediately (or at least soon) when they need one. Thus
+ * they have a higher probability to actually get a fraction
+ * of the device throughput proportional to their high
+ * weight. This is especially true with NCQ-capable drives,
+ * 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.
+ */
+ idling_boosts_thr_without_issues = idling_boosts_thr &&
+ bfqd->wr_busy_queues == 0;
+
+ /*
* There is then a case where idling must be performed not for
* throughput concerns, but to preserve service guarantees. To
* introduce it, we can note that allowing the drive to
@@ -5593,7 +5652,7 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
* is necessary to preserve service guarantees.
*/
return bfq_bfqq_sync(bfqq) &&
- (idling_boosts_thr || asymmetric_scenario);
+ (idling_boosts_thr_without_issues || asymmetric_scenario);
}

/*
@@ -6801,6 +6860,7 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
* high-definition compressed
* video.
*/
+ bfqd->wr_busy_queues = 0;

/*
* Begin by assuming, optimistically, that the device is a
--
2.10.0
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