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block, bfq: inject I/O to underutilized actuators

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The main service scheme of BFQ for sync I/O is serving one sync
bfq_queue at a time, for a while. In particular, BFQ enforces this
scheme when it deems the latter necessary to boost throughput or
to preserve service guarantees. Unfortunately, when BFQ enforces
this policy, only one actuator at a time gets served for a while,
because each bfq_queue contains I/O only for one actuator. The
other actuators may remain underutilized.

Actually, BFQ may serve (inject) extra I/O, taken from other
bfq_queues, in parallel with that of the in-service queue. This
injection mechanism may provide the ground for dealing also with
the above actuator-underutilization problem. Yet BFQ does not take
the actuator load into account when choosing which queue to pick
extra I/O from. In addition, BFQ may happen to inject extra I/O
only when the in-service queue is temporarily empty.

In view of these facts, this commit extends the
injection mechanism in such a way that the latter:
(1) takes into account also the actuator load;
(2) checks such a load on each dispatch, and injects I/O for an
    underutilized actuator, if there is one and there is I/O for it.

To perform the check in (2), this commit introduces a load
threshold, currently set to 4.  A linear scan of each actuator is
performed, until an actuator is found for which the following two
conditions hold: the load of the actuator is below the threshold,
and there is at least one non-in-service queue that contains I/O
for that actuator. If such a pair (actuator, queue) is found, then
the head request of that queue is returned for dispatch, instead
of the head request of the in-service queue.

We have set the threshold, empirically, to the minimum possible
value for which an actuator is fully utilized, or close to be
fully utilized. By doing so, injected I/O 'steals' as few
drive-queue slots as possibile to the in-service queue. This
reduces as much as possible the probability that the service of
I/O from the in-service bfq_queue gets delayed because of slot
exhaustion, i.e., because all the slots of the drive queue are
filled with I/O injected from other queues (NCQ provides for 32
slots).

This new mechanism also counters actuator underutilization in the
case of asymmetric configurations of bfq_queues. Namely if there
are few bfq_queues containing I/O for some actuators and many
bfq_queues containing I/O for other actuators. Or if the
bfq_queues containing I/O for some actuators have lower weights
than the other bfq_queues.

Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Davide Zini <davidezini2@gmail.com>
Link: https://lore.kernel.org/r/20230103145503.71712-8-paolo.valente@linaro.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Davide Zini 2023-01-03 15:55:02 +01:00 committed by Jens Axboe
parent 4fdb3b9f2a
commit 2d31c684a0
4 changed files with 139 additions and 40 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
block/bfq-cgroup.c
View File

@ -706,7 +706,7 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bfq_activate_bfqq(bfqd, bfqq);
}
if (!bfqd->in_service_queue && !bfqd->rq_in_driver)
if (!bfqd->in_service_queue && !bfqd->tot_rq_in_driver)
bfq_schedule_dispatch(bfqd);
/* release extra ref taken above, bfqq may happen to be freed now */
bfq_put_queue(bfqq);

136
block/bfq-iosched.c
View File

@ -2259,9 +2259,9 @@ static void bfq_add_request(struct request *rq)
* elapsed.
*/
if (bfqq == bfqd->in_service_queue &&
(bfqd->rq_in_driver == 0 ||
(bfqd->tot_rq_in_driver == 0 ||
(bfqq->last_serv_time_ns > 0 &&
bfqd->rqs_injected && bfqd->rq_in_driver > 0)) &&
bfqd->rqs_injected && bfqd->tot_rq_in_driver > 0)) &&
time_is_before_eq_jiffies(bfqq->decrease_time_jif +
msecs_to_jiffies(10))) {
bfqd->last_empty_occupied_ns = ktime_get_ns();
@ -2285,7 +2285,7 @@ static void bfq_add_request(struct request *rq)
* will be set in case injection is performed
* on bfqq before rq is completed).
*/
if (bfqd->rq_in_driver == 0)
if (bfqd->tot_rq_in_driver == 0)
bfqd->rqs_injected = false;
}
}
@ -2650,11 +2650,14 @@ void bfq_end_wr_async_queues(struct bfq_data *bfqd,
static void bfq_end_wr(struct bfq_data *bfqd)
{
struct bfq_queue *bfqq;
int i;
spin_lock_irq(&bfqd->lock);
list_for_each_entry(bfqq, &bfqd->active_list, bfqq_list)
bfq_bfqq_end_wr(bfqq);
for (i = 0; i < bfqd->num_actuators; i++) {
list_for_each_entry(bfqq, &bfqd->active_list[i], bfqq_list)
bfq_bfqq_end_wr(bfqq);
}
list_for_each_entry(bfqq, &bfqd->idle_list, bfqq_list)
bfq_bfqq_end_wr(bfqq);
bfq_end_wr_async(bfqd);
@ -3611,13 +3614,13 @@ static void bfq_update_peak_rate(struct bfq_data *bfqd, struct request *rq)
* - start a new observation interval with this dispatch
*/
if (now_ns - bfqd->last_dispatch > 100*NSEC_PER_MSEC &&
bfqd->rq_in_driver == 0)
bfqd->tot_rq_in_driver == 0)
goto update_rate_and_reset;
/* Update sampling information */
bfqd->peak_rate_samples++;
if ((bfqd->rq_in_driver > 0 ||
if ((bfqd->tot_rq_in_driver > 0 ||
now_ns - bfqd->last_completion < BFQ_MIN_TT)
&& !BFQ_RQ_SEEKY(bfqd, bfqd->last_position, rq))
bfqd->sequential_samples++;
@ -3882,10 +3885,8 @@ static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
return false;
return (bfqq->wr_coeff > 1 &&
(bfqd->wr_busy_queues <
tot_busy_queues ||
bfqd->rq_in_driver >=
bfqq->dispatched + 4)) ||
(bfqd->wr_busy_queues < tot_busy_queues ||
bfqd->tot_rq_in_driver >= bfqq->dispatched + 4)) ||
bfq_asymmetric_scenario(bfqd, bfqq) ||
tot_busy_queues == 1;
}
@ -4656,6 +4657,8 @@ bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
{
struct bfq_queue *bfqq, *in_serv_bfqq = bfqd->in_service_queue;
unsigned int limit = in_serv_bfqq->inject_limit;
int i;
/*
* If
* - bfqq is not weight-raised and therefore does not carry
@ -4687,7 +4690,7 @@ bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
)
limit = 1;
if (bfqd->rq_in_driver >= limit)
if (bfqd->tot_rq_in_driver >= limit)
return NULL;
/*
@ -4702,11 +4705,12 @@ bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
* (and re-added only if it gets new requests, but then it
* is assigned again enough budget for its new backlog).
*/
list_for_each_entry(bfqq, &bfqd->active_list, bfqq_list)
if (!RB_EMPTY_ROOT(&bfqq->sort_list) &&
(in_serv_always_inject || bfqq->wr_coeff > 1) &&
bfq_serv_to_charge(bfqq->next_rq, bfqq) <=
bfq_bfqq_budget_left(bfqq)) {
for (i = 0; i < bfqd->num_actuators; i++) {
list_for_each_entry(bfqq, &bfqd->active_list[i], bfqq_list)
if (!RB_EMPTY_ROOT(&bfqq->sort_list) &&
(in_serv_always_inject || bfqq->wr_coeff > 1) &&
bfq_serv_to_charge(bfqq->next_rq, bfqq) <=
bfq_bfqq_budget_left(bfqq)) {
/*
* Allow for only one large in-flight request
* on non-rotational devices, for the
@ -4731,22 +4735,69 @@ bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
else
limit = in_serv_bfqq->inject_limit;
if (bfqd->rq_in_driver < limit) {
if (bfqd->tot_rq_in_driver < limit) {
bfqd->rqs_injected = true;
return bfqq;
}
}
}
return NULL;
}
static struct bfq_queue *
bfq_find_active_bfqq_for_actuator(struct bfq_data *bfqd, int idx)
{
struct bfq_queue *bfqq;
if (bfqd->in_service_queue &&
bfqd->in_service_queue->actuator_idx == idx)
return bfqd->in_service_queue;
list_for_each_entry(bfqq, &bfqd->active_list[idx], bfqq_list) {
if (!RB_EMPTY_ROOT(&bfqq->sort_list) &&
bfq_serv_to_charge(bfqq->next_rq, bfqq) <=
bfq_bfqq_budget_left(bfqq)) {
return bfqq;
}
}
return NULL;
}
/*
* Perform a linear scan of each actuator, until an actuator is found
* for which the following two conditions hold: the load of the
* actuator is below the threshold (see comments on actuator_load_threshold
* for details), and there is a queue that contains I/O for that
* actuator. On success, return that queue.
*/
static struct bfq_queue *
bfq_find_bfqq_for_underused_actuator(struct bfq_data *bfqd)
{
int i;
for (i = 0 ; i < bfqd->num_actuators; i++) {
if (bfqd->rq_in_driver[i] < bfqd->actuator_load_threshold) {
struct bfq_queue *bfqq =
bfq_find_active_bfqq_for_actuator(bfqd, i);
if (bfqq)
return bfqq;
}
}
return NULL;
}
/*
* Select a queue for service. If we have a current queue in service,
* check whether to continue servicing it, or retrieve and set a new one.
*/
static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
{
struct bfq_queue *bfqq;
struct bfq_queue *bfqq, *inject_bfqq;
struct request *next_rq;
enum bfqq_expiration reason = BFQQE_BUDGET_TIMEOUT;
@ -4768,6 +4819,15 @@ static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
goto expire;
check_queue:
/*
* If some actuator is underutilized, but the in-service
* queue does not contain I/O for that actuator, then try to
* inject I/O for that actuator.
*/
inject_bfqq = bfq_find_bfqq_for_underused_actuator(bfqd);
if (inject_bfqq && inject_bfqq != bfqq)
return inject_bfqq;
/*
* This loop is rarely executed more than once. Even when it
* happens, it is much more convenient to re-execute this loop
@ -5123,11 +5183,11 @@ static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
/*
* We exploit the bfq_finish_requeue_request hook to
* decrement rq_in_driver, but
* decrement tot_rq_in_driver, but
* bfq_finish_requeue_request will not be invoked on
* this request. So, to avoid unbalance, just start
* this request, without incrementing rq_in_driver. As
* a negative consequence, rq_in_driver is deceptively
* this request, without incrementing tot_rq_in_driver. As
* a negative consequence, tot_rq_in_driver is deceptively
* lower than it should be while this request is in
* service. This may cause bfq_schedule_dispatch to be
* invoked uselessly.
@ -5136,7 +5196,7 @@ static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
* bfq_finish_requeue_request hook, if defined, is
* probably invoked also on this request. So, by
* exploiting this hook, we could 1) increment
* rq_in_driver here, and 2) decrement it in
* tot_rq_in_driver here, and 2) decrement it in
* bfq_finish_requeue_request. Such a solution would
* let the value of the counter be always accurate,
* but it would entail using an extra interface
@ -5165,7 +5225,7 @@ static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
* Of course, serving one request at a time may cause loss of
* throughput.
*/
if (bfqd->strict_guarantees && bfqd->rq_in_driver > 0)
if (bfqd->strict_guarantees && bfqd->tot_rq_in_driver > 0)
goto exit;
bfqq = bfq_select_queue(bfqd);
@ -5176,7 +5236,8 @@ static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
if (rq) {
inc_in_driver_start_rq:
bfqd->rq_in_driver++;
bfqd->rq_in_driver[bfqq->actuator_idx]++;
bfqd->tot_rq_in_driver++;
start_rq:
rq->rq_flags |= RQF_STARTED;
}
@ -6243,7 +6304,7 @@ static void bfq_update_hw_tag(struct bfq_data *bfqd)
struct bfq_queue *bfqq = bfqd->in_service_queue;
bfqd->max_rq_in_driver = max_t(int, bfqd->max_rq_in_driver,
bfqd->rq_in_driver);
bfqd->tot_rq_in_driver);
if (bfqd->hw_tag == 1)
return;
@ -6254,7 +6315,7 @@ static void bfq_update_hw_tag(struct bfq_data *bfqd)
* sum is not exact, as it's not taking into account deactivated
* requests.
*/
if (bfqd->rq_in_driver + bfqd->queued <= BFQ_HW_QUEUE_THRESHOLD)
if (bfqd->tot_rq_in_driver + bfqd->queued <= BFQ_HW_QUEUE_THRESHOLD)
return;
/*
@ -6265,7 +6326,7 @@ static void bfq_update_hw_tag(struct bfq_data *bfqd)
if (bfqq && bfq_bfqq_has_short_ttime(bfqq) &&
bfqq->dispatched + bfqq->queued[0] + bfqq->queued[1] <
BFQ_HW_QUEUE_THRESHOLD &&
bfqd->rq_in_driver < BFQ_HW_QUEUE_THRESHOLD)
bfqd->tot_rq_in_driver < BFQ_HW_QUEUE_THRESHOLD)
return;
if (bfqd->hw_tag_samples++ < BFQ_HW_QUEUE_SAMPLES)
@ -6286,7 +6347,8 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
bfq_update_hw_tag(bfqd);
bfqd->rq_in_driver--;
bfqd->rq_in_driver[bfqq->actuator_idx]--;
bfqd->tot_rq_in_driver--;
bfqq->dispatched--;
if (!bfqq->dispatched && !bfq_bfqq_busy(bfqq)) {
@ -6406,7 +6468,7 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
BFQQE_NO_MORE_REQUESTS);
}
if (!bfqd->rq_in_driver)
if (!bfqd->tot_rq_in_driver)
bfq_schedule_dispatch(bfqd);
}
@ -6537,13 +6599,13 @@ static void bfq_update_inject_limit(struct bfq_data *bfqd,
* conditions to do it, or we can lower the last base value
* computed.
*
* NOTE: (bfqd->rq_in_driver == 1) means that there is no I/O
* NOTE: (bfqd->tot_rq_in_driver == 1) means that there is no I/O
* request in flight, because this function is in the code
* path that handles the completion of a request of bfqq, and,
* in particular, this function is executed before
* bfqd->rq_in_driver is decremented in such a code path.
* bfqd->tot_rq_in_driver is decremented in such a code path.
*/
if ((bfqq->last_serv_time_ns == 0 && bfqd->rq_in_driver == 1) ||
if ((bfqq->last_serv_time_ns == 0 && bfqd->tot_rq_in_driver == 1) ||
tot_time_ns < bfqq->last_serv_time_ns) {
if (bfqq->last_serv_time_ns == 0) {
/*
@ -6553,7 +6615,7 @@ static void bfq_update_inject_limit(struct bfq_data *bfqd,
bfqq->inject_limit = max_t(unsigned int, 1, old_limit);
}
bfqq->last_serv_time_ns = tot_time_ns;
} else if (!bfqd->rqs_injected && bfqd->rq_in_driver == 1)
} else if (!bfqd->rqs_injected && bfqd->tot_rq_in_driver == 1)
/*
* No I/O injected and no request still in service in
* the drive: these are the exact conditions for
@ -7208,7 +7270,8 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
bfqd->num_groups_with_pending_reqs = 0;
#endif
INIT_LIST_HEAD(&bfqd->active_list);
INIT_LIST_HEAD(&bfqd->active_list[0]);
INIT_LIST_HEAD(&bfqd->active_list[1]);
INIT_LIST_HEAD(&bfqd->idle_list);
INIT_HLIST_HEAD(&bfqd->burst_list);
@ -7253,6 +7316,9 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
ref_wr_duration[blk_queue_nonrot(bfqd->queue)];
bfqd->peak_rate = ref_rate[blk_queue_nonrot(bfqd->queue)] * 2 / 3;
/* see comments on the definition of next field inside bfq_data */
bfqd->actuator_load_threshold = 4;
spin_lock_init(&bfqd->lock);
/*

39
block/bfq-iosched.h
View File

@ -590,7 +590,12 @@ struct bfq_data {
/* number of queued requests */
int queued;
/* number of requests dispatched and waiting for completion */
int rq_in_driver;
int tot_rq_in_driver;
/*
* number of requests dispatched and waiting for completion
* for each actuator
*/
int rq_in_driver[BFQ_MAX_ACTUATORS];
/* true if the device is non rotational and performs queueing */
bool nonrot_with_queueing;
@ -684,8 +689,13 @@ struct bfq_data {
/* maximum budget allotted to a bfq_queue before rescheduling */
int bfq_max_budget;
/* list of all the bfq_queues active on the device */
struct list_head active_list;
/*
* List of all the bfq_queues active for a specific actuator
* on the device. Keeping active queues separate on a
* per-actuator basis helps implementing per-actuator
* injection more efficiently.
*/
struct list_head active_list[BFQ_MAX_ACTUATORS];
/* list of all the bfq_queues idle on the device */
struct list_head idle_list;
@ -821,6 +831,29 @@ struct bfq_data {
sector_t sector[BFQ_MAX_ACTUATORS];
sector_t nr_sectors[BFQ_MAX_ACTUATORS];
struct blk_independent_access_range ia_ranges[BFQ_MAX_ACTUATORS];
/*
* If the number of I/O requests queued in the device for a
* given actuator is below next threshold, then the actuator
* is deemed as underutilized. If this condition is found to
* hold for some actuator upon a dispatch, but (i) the
* in-service queue does not contain I/O for that actuator,
* while (ii) some other queue does contain I/O for that
* actuator, then the head I/O request of the latter queue is
* returned (injected), instead of the head request of the
* currently in-service queue.
*
* We set the threshold, empirically, to the minimum possible
* value for which an actuator is fully utilized, or close to
* be fully utilized. By doing so, injected I/O 'steals' as
* few drive-queue slots as possibile to the in-service
* queue. This reduces as much as possible the probability
* that the service of I/O from the in-service bfq_queue gets
* delayed because of slot exhaustion, i.e., because all the
* slots of the drive queue are filled with I/O injected from
* other queues (NCQ provides for 32 slots).
*/
unsigned int actuator_load_threshold;
};
enum bfqq_state_flags {

2
block/bfq-wf2q.c
View File

@ -493,7 +493,7 @@ static void bfq_active_insert(struct bfq_service_tree *st,
bfq_update_active_tree(node);
if (bfqq)
list_add(&bfqq->bfqq_list, &bfqq->bfqd->active_list);
list_add(&bfqq->bfqq_list, &bfqq->bfqd->active_list[bfqq->actuator_idx]);
bfq_inc_active_entities(entity);
}