blk-throttle: add downgrade logic

When queue state machine is in LIMIT_MAX state, but a cgroup is below
its low limit for some time, the queue should be downgraded to lower
state as one cgroup's low limit isn't met.
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-throttle.c

@@ -140,6 +140,13 @@ struct throtl_grp {
 	/* Number of bio's dispatched in current slice */
 	unsigned int io_disp[2];
 
+	unsigned long last_low_overflow_time[2];
+
+	uint64_t last_bytes_disp[2];
+	unsigned int last_io_disp[2];
+
+	unsigned long last_check_time;
+
 	/* When did we start a new slice */
 	unsigned long slice_start[2];
 	unsigned long slice_end[2];
@@ -159,6 +166,9 @@ struct throtl_data
 	struct work_struct dispatch_work;
 	unsigned int limit_index;
 	bool limit_valid[LIMIT_CNT];
+
+	unsigned long low_upgrade_time;
+	unsigned long low_downgrade_time;
 };
 
 static void throtl_pending_timer_fn(unsigned long arg);
@@ -898,6 +908,8 @@ static void throtl_charge_bio(struct throtl_grp *tg, struct bio *bio)
 	/* Charge the bio to the group */
 	tg->bytes_disp[rw] += bio->bi_iter.bi_size;
 	tg->io_disp[rw]++;
+	tg->last_bytes_disp[rw] += bio->bi_iter.bi_size;
+	tg->last_io_disp[rw]++;
 
 	/*
 	 * BIO_THROTTLED is used to prevent the same bio to be throttled
@@ -1527,6 +1539,45 @@ static struct blkcg_policy blkcg_policy_throtl = {
 	.pd_free_fn		= throtl_pd_free,
 };
 
+static unsigned long __tg_last_low_overflow_time(struct throtl_grp *tg)
+{
+	unsigned long rtime = jiffies, wtime = jiffies;
+
+	if (tg->bps[READ][LIMIT_LOW] || tg->iops[READ][LIMIT_LOW])
+		rtime = tg->last_low_overflow_time[READ];
+	if (tg->bps[WRITE][LIMIT_LOW] || tg->iops[WRITE][LIMIT_LOW])
+		wtime = tg->last_low_overflow_time[WRITE];
+	return min(rtime, wtime);
+}
+
+/* tg should not be an intermediate node */
+static unsigned long tg_last_low_overflow_time(struct throtl_grp *tg)
+{
+	struct throtl_service_queue *parent_sq;
+	struct throtl_grp *parent = tg;
+	unsigned long ret = __tg_last_low_overflow_time(tg);
+
+	while (true) {
+		parent_sq = parent->service_queue.parent_sq;
+		parent = sq_to_tg(parent_sq);
+		if (!parent)
+			break;
+
+		/*
+		 * The parent doesn't have low limit, it always reaches low
+		 * limit. Its overflow time is useless for children
+		 */
+		if (!parent->bps[READ][LIMIT_LOW] &&
+		    !parent->iops[READ][LIMIT_LOW] &&
+		    !parent->bps[WRITE][LIMIT_LOW] &&
+		    !parent->iops[WRITE][LIMIT_LOW])
+			continue;
+		if (time_after(__tg_last_low_overflow_time(parent), ret))
+			ret = __tg_last_low_overflow_time(parent);
+	}
+	return ret;
+}
+
 static bool throtl_tg_can_upgrade(struct throtl_grp *tg)
 {
 	struct throtl_service_queue *sq = &tg->service_queue;
@@ -1570,6 +1621,9 @@ static bool throtl_can_upgrade(struct throtl_data *td,
 	if (td->limit_index != LIMIT_LOW)
 		return false;
 
+	if (time_before(jiffies, td->low_downgrade_time + throtl_slice))
+		return false;
+
 	rcu_read_lock();
 	blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg) {
 		struct throtl_grp *tg = blkg_to_tg(blkg);
@@ -1593,6 +1647,7 @@ static void throtl_upgrade_state(struct throtl_data *td)
 	struct blkcg_gq *blkg;
 
 	td->limit_index = LIMIT_MAX;
+	td->low_upgrade_time = jiffies;
 	rcu_read_lock();
 	blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg) {
 		struct throtl_grp *tg = blkg_to_tg(blkg);
@@ -1608,6 +1663,99 @@ static void throtl_upgrade_state(struct throtl_data *td)
 	queue_work(kthrotld_workqueue, &td->dispatch_work);
 }
 
+static void throtl_downgrade_state(struct throtl_data *td, int new)
+{
+	td->limit_index = new;
+	td->low_downgrade_time = jiffies;
+}
+
+static bool throtl_tg_can_downgrade(struct throtl_grp *tg)
+{
+	struct throtl_data *td = tg->td;
+	unsigned long now = jiffies;
+
+	/*
+	 * If cgroup is below low limit, consider downgrade and throttle other
+	 * cgroups
+	 */
+	if (time_after_eq(now, td->low_upgrade_time + throtl_slice) &&
+	    time_after_eq(now, tg_last_low_overflow_time(tg) + throtl_slice))
+		return true;
+	return false;
+}
+
+static bool throtl_hierarchy_can_downgrade(struct throtl_grp *tg)
+{
+	while (true) {
+		if (!throtl_tg_can_downgrade(tg))
+			return false;
+		tg = sq_to_tg(tg->service_queue.parent_sq);
+		if (!tg || !tg_to_blkg(tg)->parent)
+			break;
+	}
+	return true;
+}
+
+static void throtl_downgrade_check(struct throtl_grp *tg)
+{
+	uint64_t bps;
+	unsigned int iops;
+	unsigned long elapsed_time;
+	unsigned long now = jiffies;
+
+	if (tg->td->limit_index != LIMIT_MAX ||
+	    !tg->td->limit_valid[LIMIT_LOW])
+		return;
+	if (!list_empty(&tg_to_blkg(tg)->blkcg->css.children))
+		return;
+	if (time_after(tg->last_check_time + throtl_slice, now))
+		return;
+
+	elapsed_time = now - tg->last_check_time;
+	tg->last_check_time = now;
+
+	if (time_before(now, tg_last_low_overflow_time(tg) + throtl_slice))
+		return;
+
+	if (tg->bps[READ][LIMIT_LOW]) {
+		bps = tg->last_bytes_disp[READ] * HZ;
+		do_div(bps, elapsed_time);
+		if (bps >= tg->bps[READ][LIMIT_LOW])
+			tg->last_low_overflow_time[READ] = now;
+	}
+
+	if (tg->bps[WRITE][LIMIT_LOW]) {
+		bps = tg->last_bytes_disp[WRITE] * HZ;
+		do_div(bps, elapsed_time);
+		if (bps >= tg->bps[WRITE][LIMIT_LOW])
+			tg->last_low_overflow_time[WRITE] = now;
+	}
+
+	if (tg->iops[READ][LIMIT_LOW]) {
+		iops = tg->last_io_disp[READ] * HZ / elapsed_time;
+		if (iops >= tg->iops[READ][LIMIT_LOW])
+			tg->last_low_overflow_time[READ] = now;
+	}
+
+	if (tg->iops[WRITE][LIMIT_LOW]) {
+		iops = tg->last_io_disp[WRITE] * HZ / elapsed_time;
+		if (iops >= tg->iops[WRITE][LIMIT_LOW])
+			tg->last_low_overflow_time[WRITE] = now;
+	}
+
+	/*
+	 * If cgroup is below low limit, consider downgrade and throttle other
+	 * cgroups
+	 */
+	if (throtl_hierarchy_can_downgrade(tg))
+		throtl_downgrade_state(tg->td, LIMIT_LOW);
+
+	tg->last_bytes_disp[READ] = 0;
+	tg->last_bytes_disp[WRITE] = 0;
+	tg->last_io_disp[READ] = 0;
+	tg->last_io_disp[WRITE] = 0;
+}
+
 bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 		    struct bio *bio)
 {
@@ -1632,12 +1780,16 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 
 again:
 	while (true) {
+		if (tg->last_low_overflow_time[rw] == 0)
+			tg->last_low_overflow_time[rw] = jiffies;
+		throtl_downgrade_check(tg);
 		/* throtl is FIFO - if bios are already queued, should queue */
 		if (sq->nr_queued[rw])
 			break;
 
 		/* if above limits, break to queue */
 		if (!tg_may_dispatch(tg, bio, NULL)) {
+			tg->last_low_overflow_time[rw] = jiffies;
 			if (throtl_can_upgrade(tg->td, tg)) {
 				throtl_upgrade_state(tg->td);
 				goto again;
@@ -1681,6 +1833,8 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 		   tg->io_disp[rw], tg_iops_limit(tg, rw),
 		   sq->nr_queued[READ], sq->nr_queued[WRITE]);
 
+	tg->last_low_overflow_time[rw] = jiffies;
+
 	bio_associate_current(bio);
 	tg->td->nr_queued[rw]++;
 	throtl_add_bio_tg(bio, qn, tg);
@@ -1791,6 +1945,8 @@ int blk_throtl_init(struct request_queue *q)
 
 	td->limit_valid[LIMIT_MAX] = true;
 	td->limit_index = LIMIT_MAX;
+	td->low_upgrade_time = jiffies;
+	td->low_downgrade_time = jiffies;
 	/* activate policy */
 	ret = blkcg_activate_policy(q, &blkcg_policy_throtl);
 	if (ret)