Message-Id: <20170803164818.10562-1-paolo.valente@linaro.org>
Date:   Thu,  3 Aug 2017 18:48:18 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ulf.hansson@...aro.org, broonie@...nel.org, lucmiccio@...il.com,
        Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX/IMPROVEMENT] block, bfq: improve and refactor throughput-boosting logic

When a queue associated with a process remains empty, there are cases
where throughput gets boosted if the device is idled to await the
arrival of a new I/O request for that queue. Currently, BFQ assumes
that one of these cases is when the device has no internal queueing
(regardless of the properties of the I/O being served). Unfortunately,
this condition has proved to be too general. So, this commit refines it
as "the device has no internal queueing and is rotational".

This refinement provides a significant throughput boost with random
I/O, on flash-based storage without internal queueing. For example, on
a HiKey board, throughput increases by up to 125%, growing, e.g., from
6.9MB/s to 15.6MB/s with two or three random readers in parallel.

This commit also refactors the code related to device idling, for the
following reason. Finding the change that provides the above large
improvement has been slightly more difficult than it had to be,
because the logic that decides whether to idle the device is still
scattered across three functions. Almost all of the logic is in the
function bfq_bfqq_may_idle, but (1) part of the decision is made in
bfq_update_idle_window, and (2) the function bfq_bfqq_must_idle may
switch off idling regardless of the output of bfq_bfqq_may_idle. In
addition, both bfq_update_idle_window and bfq_bfqq_must_idle make
their decisions as a function of parameters that are used, for similar
purposes, also in bfq_bfqq_may_idle. This commit addresses this issue
by moving all the logic into bfq_bfqq_may_idle.

Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
Signed-off-by: Luca Miccio <lucmiccio@...il.com>
---
 block/bfq-iosched.c | 144 ++++++++++++++++++++++++++++------------------------
 block/bfq-iosched.h |  12 ++---
 2 files changed, 85 insertions(+), 71 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 436b6ca..509f399 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -128,7 +128,7 @@ BFQ_BFQQ_FNS(busy);
 BFQ_BFQQ_FNS(wait_request);
 BFQ_BFQQ_FNS(non_blocking_wait_rq);
 BFQ_BFQQ_FNS(fifo_expire);
-BFQ_BFQQ_FNS(idle_window);
+BFQ_BFQQ_FNS(has_short_ttime);
 BFQ_BFQQ_FNS(sync);
 BFQ_BFQQ_FNS(IO_bound);
 BFQ_BFQQ_FNS(in_large_burst);
@@ -731,10 +731,10 @@ bfq_bfqq_resume_state(struct bfq_queue *bfqq, struct bfq_data *bfqd,
 	unsigned int old_wr_coeff = bfqq->wr_coeff;
 	bool busy = bfq_already_existing && bfq_bfqq_busy(bfqq);
 
-	if (bic->saved_idle_window)
-		bfq_mark_bfqq_idle_window(bfqq);
+	if (bic->saved_has_short_ttime)
+		bfq_mark_bfqq_has_short_ttime(bfqq);
 	else
-		bfq_clear_bfqq_idle_window(bfqq);
+		bfq_clear_bfqq_has_short_ttime(bfqq);
 
 	if (bic->saved_IO_bound)
 		bfq_mark_bfqq_IO_bound(bfqq);
@@ -2012,7 +2012,7 @@ static void bfq_bfqq_save_state(struct bfq_queue *bfqq)
 		return;
 
 	bic->saved_ttime = bfqq->ttime;
-	bic->saved_idle_window = bfq_bfqq_idle_window(bfqq);
+	bic->saved_has_short_ttime = bfq_bfqq_has_short_ttime(bfqq);
 	bic->saved_IO_bound = bfq_bfqq_IO_bound(bfqq);
 	bic->saved_in_large_burst = bfq_bfqq_in_large_burst(bfqq);
 	bic->was_in_burst_list = !hlist_unhashed(&bfqq->burst_list_node);
@@ -3038,8 +3038,8 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
 	}
 
 	bfq_log_bfqq(bfqd, bfqq,
-		"expire (%d, slow %d, num_disp %d, idle_win %d)", reason,
-		slow, bfqq->dispatched, bfq_bfqq_idle_window(bfqq));
+		"expire (%d, slow %d, num_disp %d, short_ttime %d)", reason,
+		slow, bfqq->dispatched, bfq_bfqq_has_short_ttime(bfqq));
 
 	/*
 	 * Increase, decrease or leave budget unchanged according to
@@ -3114,7 +3114,10 @@ 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, idling_boosts_thr_without_issues,
+	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;
 
@@ -3122,27 +3125,45 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
 		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;
+
+	bfqq_sequential_and_IO_bound = !BFQQ_SEEKY(bfqq) &&
+		bfq_bfqq_IO_bound(bfqq) && bfq_bfqq_has_short_ttime(bfqq);
+
+	/*
 	 * The next variable takes into account the cases where idling
 	 * boosts the throughput.
 	 *
 	 * The value of the variable is computed considering, first, that
 	 * idling is virtually always beneficial for the throughput if:
-	 * (a) the device is not NCQ-capable, or
-	 * (b) regardless of the presence of NCQ, the device is rotational
-	 *     and the request pattern for bfqq is I/O-bound and sequential.
+	 * (a) the device is not NCQ-capable and rotational, or
+	 * (b) regardless of the presence of NCQ, the device is rotational and
+	 *     the request pattern for bfqq is I/O-bound and sequential, or
+	 * (c) regardless of whether it is rotational, the device is
+	 *     not NCQ-capable and the request pattern for bfqq is
+	 *     I/O-bound and sequential.
 	 *
 	 * Secondly, and in contrast to the above item (b), idling an
 	 * NCQ-capable flash-based device would not boost the
 	 * throughput even with sequential I/O; rather it would lower
 	 * the throughput in proportion to how fast the device
 	 * is. Accordingly, the next variable is true if any of the
-	 * above conditions (a) and (b) is true, and, in particular,
-	 * happens to be false if bfqd is an NCQ-capable flash-based
-	 * device.
+	 * above conditions (a), (b) or (c) is true, and, in
+	 * particular, happens to be false if bfqd is an NCQ-capable
+	 * flash-based device.
 	 */
-	idling_boosts_thr = !bfqd->hw_tag ||
-		(!blk_queue_nonrot(bfqd->queue) && bfq_bfqq_IO_bound(bfqq) &&
-		 bfq_bfqq_idle_window(bfqq));
+	idling_boosts_thr = rot_without_queueing ||
+		((!blk_queue_nonrot(bfqd->queue) || !bfqd->hw_tag) &&
+		 bfqq_sequential_and_IO_bound);
 
 	/*
 	 * The value of the next variable,
@@ -3313,16 +3334,13 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
 		asymmetric_scenario && !bfq_bfqq_in_large_burst(bfqq);
 
 	/*
-	 * We have now all the components we need to compute the return
-	 * value of the function, which is true only if both the following
-	 * conditions hold:
-	 * 1) bfqq is sync, because idling make sense only for sync queues;
-	 * 2) idling either boosts the throughput (without issues), or
-	 *    is necessary to preserve service guarantees.
+	 * We have now all the 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 bfq_bfqq_sync(bfqq) &&
-		(idling_boosts_thr_without_issues ||
-		 idling_needed_for_service_guarantees);
+	return idling_boosts_thr_without_issues ||
+		idling_needed_for_service_guarantees;
 }
 
 /*
@@ -3338,10 +3356,7 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq)
  */
 static bool bfq_bfqq_must_idle(struct bfq_queue *bfqq)
 {
-	struct bfq_data *bfqd = bfqq->bfqd;
-
-	return RB_EMPTY_ROOT(&bfqq->sort_list) && bfqd->bfq_slice_idle != 0 &&
-	       bfq_bfqq_may_idle(bfqq);
+	return RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_may_idle(bfqq);
 }
 
 /*
@@ -3783,7 +3798,6 @@ bfq_set_next_ioprio_data(struct bfq_queue *bfqq, struct bfq_io_cq *bic)
 	case IOPRIO_CLASS_IDLE:
 		bfqq->new_ioprio_class = IOPRIO_CLASS_IDLE;
 		bfqq->new_ioprio = 7;
-		bfq_clear_bfqq_idle_window(bfqq);
 		break;
 	}
 
@@ -3843,8 +3857,14 @@ static void bfq_init_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 		bfq_set_next_ioprio_data(bfqq, bic);
 
 	if (is_sync) {
+		/*
+		 * No need to mark as has_short_ttime if in
+		 * idle_class, because no device idling is performed
+		 * for queues in idle class
+		 */
 		if (!bfq_class_idle(bfqq))
-			bfq_mark_bfqq_idle_window(bfqq);
+			/* tentatively mark as has_short_ttime */
+			bfq_mark_bfqq_has_short_ttime(bfqq);
 		bfq_mark_bfqq_sync(bfqq);
 		bfq_mark_bfqq_just_created(bfqq);
 	} else
@@ -3985,18 +4005,19 @@ bfq_update_io_seektime(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 		 blk_rq_sectors(rq) < BFQQ_SECT_THR_NONROT);
 }
 
-/*
- * Disable idle window if the process thinks too long or seeks so much that
- * it doesn't matter.
- */
-static void bfq_update_idle_window(struct bfq_data *bfqd,
-				   struct bfq_queue *bfqq,
-				   struct bfq_io_cq *bic)
+static void bfq_update_has_short_ttime(struct bfq_data *bfqd,
+				       struct bfq_queue *bfqq,
+				       struct bfq_io_cq *bic)
 {
-	int enable_idle;
+	bool has_short_ttime = true;
 
-	/* Don't idle for async or idle io prio class. */
-	if (!bfq_bfqq_sync(bfqq) || bfq_class_idle(bfqq))
+	/*
+	 * No need to update has_short_ttime if bfqq is async or in
+	 * idle io prio class, or if bfq_slice_idle is zero, because
+	 * no device idling is performed for bfqq in this case.
+	 */
+	if (!bfq_bfqq_sync(bfqq) || bfq_class_idle(bfqq) ||
+	    bfqd->bfq_slice_idle == 0)
 		return;
 
 	/* Idle window just restored, statistics are meaningless. */
@@ -4004,27 +4025,22 @@ static void bfq_update_idle_window(struct bfq_data *bfqd,
 				     bfqd->bfq_wr_min_idle_time))
 		return;
 
-	enable_idle = bfq_bfqq_idle_window(bfqq);
-
+	/* Think time is infinite if no process is linked to
+	 * bfqq. Otherwise check average think time to
+	 * decide whether to mark as has_short_ttime
+	 */
 	if (atomic_read(&bic->icq.ioc->active_ref) == 0 ||
-	    bfqd->bfq_slice_idle == 0 ||
-		(bfqd->hw_tag && BFQQ_SEEKY(bfqq) &&
-			bfqq->wr_coeff == 1))
-		enable_idle = 0;
-	else if (bfq_sample_valid(bfqq->ttime.ttime_samples)) {
-		if (bfqq->ttime.ttime_mean > bfqd->bfq_slice_idle &&
-			bfqq->wr_coeff == 1)
-			enable_idle = 0;
-		else
-			enable_idle = 1;
-	}
-	bfq_log_bfqq(bfqd, bfqq, "update_idle_window: enable_idle %d",
-		enable_idle);
+	    (bfq_sample_valid(bfqq->ttime.ttime_samples) &&
+	     bfqq->ttime.ttime_mean > bfqd->bfq_slice_idle))
+		has_short_ttime = false;
+
+	bfq_log_bfqq(bfqd, bfqq, "update_has_short_ttime: has_short_ttime %d",
+		     has_short_ttime);
 
-	if (enable_idle)
-		bfq_mark_bfqq_idle_window(bfqq);
+	if (has_short_ttime)
+		bfq_mark_bfqq_has_short_ttime(bfqq);
 	else
-		bfq_clear_bfqq_idle_window(bfqq);
+		bfq_clear_bfqq_has_short_ttime(bfqq);
 }
 
 /*
@@ -4040,14 +4056,12 @@ static void bfq_rq_enqueued(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 		bfqq->meta_pending++;
 
 	bfq_update_io_thinktime(bfqd, bfqq);
+	bfq_update_has_short_ttime(bfqd, bfqq, bic);
 	bfq_update_io_seektime(bfqd, bfqq, rq);
-	if (bfqq->entity.service > bfq_max_budget(bfqd) / 8 ||
-	    !BFQQ_SEEKY(bfqq))
-		bfq_update_idle_window(bfqd, bfqq, bic);
 
 	bfq_log_bfqq(bfqd, bfqq,
-		     "rq_enqueued: idle_window=%d (seeky %d)",
-		     bfq_bfqq_idle_window(bfqq), BFQQ_SEEKY(bfqq));
+		     "rq_enqueued: has_short_ttime=%d (seeky %d)",
+		     bfq_bfqq_has_short_ttime(bfqq), BFQQ_SEEKY(bfqq));
 
 	bfqq->last_request_pos = blk_rq_pos(rq) + blk_rq_sectors(rq);
 
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 63e771a..53e2907 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -348,11 +348,11 @@ struct bfq_io_cq {
 	uint64_t blkcg_serial_nr; /* the current blkcg serial */
 #endif
 	/*
-	 * Snapshot of the idle window before merging; taken to
-	 * remember this value while the queue is merged, so as to be
-	 * able to restore it in case of split.
+	 * Snapshot of the has_short_time flag before merging; taken
+	 * to remember its value while the queue is merged, so as to
+	 * be able to restore it in case of split.
 	 */
-	bool saved_idle_window;
+	bool saved_has_short_ttime;
 	/*
 	 * Same purpose as the previous two fields for the I/O bound
 	 * classification of a queue.
@@ -626,7 +626,7 @@ enum bfqq_state_flags {
 				     * without idling the device
 				     */
 	BFQQF_fifo_expire,	/* FIFO checked in this slice */
-	BFQQF_idle_window,	/* slice idling enabled */
+	BFQQF_has_short_ttime,	/* queue has a short think time */
 	BFQQF_sync,		/* synchronous queue */
 	BFQQF_IO_bound,		/*
 				 * bfqq has timed-out at least once
@@ -655,7 +655,7 @@ BFQ_BFQQ_FNS(busy);
 BFQ_BFQQ_FNS(wait_request);
 BFQ_BFQQ_FNS(non_blocking_wait_rq);
 BFQ_BFQQ_FNS(fifo_expire);
-BFQ_BFQQ_FNS(idle_window);
+BFQ_BFQQ_FNS(has_short_ttime);
 BFQ_BFQQ_FNS(sync);
 BFQ_BFQQ_FNS(IO_bound);
 BFQ_BFQQ_FNS(in_large_burst);
-- 
2.10.0

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