lightnvm: pblk: redesign GC algorithm

At the moment, in order to get enough read parallelism, we have recycled
several lines at the same time. This approach has proven not to work
well when reaching capacity, since we end up mixing valid data from all
lines, thus not maintaining a sustainable free/recycled line ratio.

The new design, relies on a two level workqueue mechanism. In the first
level, we read the metadata for a number of lines based on the GC list
they reside on (this is governed by the number of valid sectors in each
line). In the second level, we recycle a single line at a time. Here, we
issue reads in parallel, while a single GC write thread places data in
the write buffer. This design allows to (i) only move data from one line
at a time, thus maintaining a sane free/recycled ration and (ii)
maintain the GC writer busy with recycled data.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 257 insertions, 182 deletions

diff --git a/drivers/lightnvm/pblk-gc.c b/drivers/lightnvm/pblk-gc.c
index f811e4ca63f4..1d289242ab92 100644
--- a/drivers/lightnvm/pblk-gc.c
+++ b/drivers/lightnvm/pblk-gc.c
@@ -21,7 +21,6 @@
 static void pblk_gc_free_gc_rq(struct pblk_gc_rq *gc_rq)
 {
 	kfree(gc_rq->data);
-	kfree(gc_rq->lba_list);
 	kfree(gc_rq);
 }
 
@@ -37,10 +36,8 @@ static int pblk_gc_write(struct pblk *pblk)
 		return 1;
 	}
 
-	list_for_each_entry_safe(gc_rq, tgc_rq, &gc->w_list, list) {
-		list_move_tail(&gc_rq->list, &w_list);
-		gc->w_entries--;
-	}
+	list_cut_position(&w_list, &gc->w_list, gc->w_list.prev);
+	gc->w_entries = 0;
 	spin_unlock(&gc->w_lock);
 
 	list_for_each_entry_safe(gc_rq, tgc_rq, &w_list, list) {
@@ -48,9 +45,8 @@ static int pblk_gc_write(struct pblk *pblk)
 				gc_rq->nr_secs, gc_rq->secs_to_gc,
 				gc_rq->line, PBLK_IOTYPE_GC);
 
-		kref_put(&gc_rq->line->ref, pblk_line_put);
-
 		list_del(&gc_rq->list);
+		kref_put(&gc_rq->line->ref, pblk_line_put);
 		pblk_gc_free_gc_rq(gc_rq);
 	}
 
@@ -66,52 +62,41 @@ static void pblk_gc_writer_kick(struct pblk_gc *gc)
  * Responsible for managing all memory related to a gc request. Also in case of
  * failure
  */
-static int pblk_gc_move_valid_secs(struct pblk *pblk, struct pblk_line *line,
-				   u64 *lba_list, unsigned int nr_secs)
+static int pblk_gc_move_valid_secs(struct pblk *pblk, struct pblk_gc_rq *gc_rq)
 {
 	struct nvm_tgt_dev *dev = pblk->dev;
 	struct nvm_geo *geo = &dev->geo;
 	struct pblk_gc *gc = &pblk->gc;
-	struct pblk_gc_rq *gc_rq;
+	struct pblk_line *line = gc_rq->line;
 	void *data;
 	unsigned int secs_to_gc;
-	int ret = NVM_IO_OK;
+	int ret = 0;
 
-	data = kmalloc(nr_secs * geo->sec_size, GFP_KERNEL);
+	data = kmalloc(gc_rq->nr_secs * geo->sec_size, GFP_KERNEL);
 	if (!data) {
-		ret = NVM_IO_ERR;
-		goto free_lba_list;
+		ret = -ENOMEM;
+		goto out;
 	}
 
 	/* Read from GC victim block */
-	if (pblk_submit_read_gc(pblk, lba_list, data, nr_secs,
+	if (pblk_submit_read_gc(pblk, gc_rq->lba_list, data, gc_rq->nr_secs,
 							&secs_to_gc, line)) {
-		ret = NVM_IO_ERR;
+		ret = -EFAULT;
 		goto free_data;
 	}
 
 	if (!secs_to_gc)
-		goto free_data;
-
-	gc_rq = kmalloc(sizeof(struct pblk_gc_rq), GFP_KERNEL);
-	if (!gc_rq) {
-		ret = NVM_IO_ERR;
-		goto free_data;
-	}
+		goto free_rq;
 
-	gc_rq->line = line;
 	gc_rq->data = data;
-	gc_rq->lba_list = lba_list;
-	gc_rq->nr_secs = nr_secs;
 	gc_rq->secs_to_gc = secs_to_gc;
 
-	kref_get(&line->ref);
-
 retry:
 	spin_lock(&gc->w_lock);
-	if (gc->w_entries > 256) {
+	if (gc->w_entries >= PBLK_GC_W_QD) {
 		spin_unlock(&gc->w_lock);
-		usleep_range(256, 1024);
+		pblk_gc_writer_kick(&pblk->gc);
+		usleep_range(128, 256);
 		goto retry;
 	}
 	gc->w_entries++;
@@ -120,13 +105,14 @@ retry:
 
 	pblk_gc_writer_kick(&pblk->gc);
 
-	return NVM_IO_OK;
+	return 0;
 
+free_rq:
+	kfree(gc_rq);
 free_data:
 	kfree(data);
-free_lba_list:
-	kfree(lba_list);
-
+out:
+	kref_put(&line->ref, pblk_line_put);
 	return ret;
 }
 
@@ -150,20 +136,52 @@ static void pblk_put_line_back(struct pblk *pblk, struct pblk_line *line)
 
 static void pblk_gc_line_ws(struct work_struct *work)
 {
+	struct pblk_line_ws *line_rq_ws = container_of(work,
+						struct pblk_line_ws, ws);
+	struct pblk *pblk = line_rq_ws->pblk;
+	struct pblk_gc *gc = &pblk->gc;
+	struct pblk_line *line = line_rq_ws->line;
+	struct pblk_gc_rq *gc_rq = line_rq_ws->priv;
+
+	up(&gc->gc_sem);
+
+	if (pblk_gc_move_valid_secs(pblk, gc_rq)) {
+		pr_err("pblk: could not GC all sectors: line:%d (%d/%d)\n",
+						line->id, *line->vsc,
+						gc_rq->nr_secs);
+	}
+
+	mempool_free(line_rq_ws, pblk->line_ws_pool);
+}
+
+static void pblk_gc_line_prepare_ws(struct work_struct *work)
+{
 	struct pblk_line_ws *line_ws = container_of(work, struct pblk_line_ws,
 									ws);
 	struct pblk *pblk = line_ws->pblk;
-	struct pblk_line_mgmt *l_mg = &pblk->l_mg;
 	struct pblk_line *line = line_ws->line;
+	struct pblk_line_mgmt *l_mg = &pblk->l_mg;
 	struct pblk_line_meta *lm = &pblk->lm;
-	struct line_emeta *emeta_buf = line_ws->priv;
+	struct pblk_gc *gc = &pblk->gc;
+	struct line_emeta *emeta_buf;
+	struct pblk_line_ws *line_rq_ws;
+	struct pblk_gc_rq *gc_rq;
 	__le64 *lba_list;
-	u64 *gc_list;
-	int sec_left;
-	int nr_ppas, bit;
-	int put_line = 1;
+	int sec_left, nr_secs, bit;
+	int ret;
 
-	pr_debug("pblk: line '%d' being reclaimed for GC\n", line->id);
+	emeta_buf = pblk_malloc(lm->emeta_len[0], l_mg->emeta_alloc_type,
+								GFP_KERNEL);
+	if (!emeta_buf) {
+		pr_err("pblk: cannot use GC emeta\n");
+		return;
+	}
+
+	ret = pblk_line_read_emeta(pblk, line, emeta_buf);
+	if (ret) {
+		pr_err("pblk: line %d read emeta failed (%d)\n", line->id, ret);
+		goto fail_free_emeta;
+	}
 
 	/* If this read fails, it means that emeta is corrupted. For now, leave
 	 * the line untouched. TODO: Implement a recovery routine that scans and
@@ -172,119 +190,124 @@ static void pblk_gc_line_ws(struct work_struct *work)
 	lba_list = pblk_recov_get_lba_list(pblk, emeta_buf);
 	if (!lba_list) {
 		pr_err("pblk: could not interpret emeta (line %d)\n", line->id);
-		goto out;
-	}
-
-	spin_lock(&line->lock);
-	sec_left = le32_to_cpu(*line->vsc);
-	if (!sec_left) {
-		/* Lines are erased before being used (l_mg->data_/log_next) */
-		spin_unlock(&line->lock);
-		goto out;
+		goto fail_free_emeta;
 	}
-	spin_unlock(&line->lock);
 
+	sec_left = pblk_line_vsc(line);
 	if (sec_left < 0) {
 		pr_err("pblk: corrupted GC line (%d)\n", line->id);
-		put_line = 0;
-		pblk_put_line_back(pblk, line);
-		goto out;
+		goto fail_free_emeta;
 	}
 
 	bit = -1;
 next_rq:
-	gc_list = kmalloc_array(pblk->max_write_pgs, sizeof(u64), GFP_KERNEL);
-	if (!gc_list) {
-		put_line = 0;
-		pblk_put_line_back(pblk, line);
-		goto out;
-	}
+	gc_rq = kmalloc(sizeof(struct pblk_gc_rq), GFP_KERNEL);
+	if (!gc_rq)
+		goto fail_free_emeta;
 
-	nr_ppas = 0;
+	nr_secs = 0;
 	do {
 		bit = find_next_zero_bit(line->invalid_bitmap, lm->sec_per_line,
 								bit + 1);
 		if (bit > line->emeta_ssec)
 			break;
 
-		gc_list[nr_ppas++] = le64_to_cpu(lba_list[bit]);
-	} while (nr_ppas < pblk->max_write_pgs);
+		gc_rq->lba_list[nr_secs++] = le64_to_cpu(lba_list[bit]);
+	} while (nr_secs < pblk->max_write_pgs);
 
-	if (unlikely(!nr_ppas)) {
-		kfree(gc_list);
+	if (unlikely(!nr_secs)) {
+		kfree(gc_rq);
 		goto out;
 	}
 
-	if (pblk_gc_move_valid_secs(pblk, line, gc_list, nr_ppas)) {
-		pr_err("pblk: could not GC all sectors: line:%d (%d/%d/%d)\n",
-						line->id, *line->vsc,
-						nr_ppas, nr_ppas);
-		put_line = 0;
-		pblk_put_line_back(pblk, line);
-		goto out;
-	}
+	gc_rq->nr_secs = nr_secs;
+	gc_rq->line = line;
+
+	line_rq_ws = mempool_alloc(pblk->line_ws_pool, GFP_KERNEL);
+	if (!line_rq_ws)
+		goto fail_free_gc_rq;
+
+	line_rq_ws->pblk = pblk;
+	line_rq_ws->line = line;
+	line_rq_ws->priv = gc_rq;
+
+	down(&gc->gc_sem);
+	kref_get(&line->ref);
+
+	INIT_WORK(&line_rq_ws->ws, pblk_gc_line_ws);
+	queue_work(gc->gc_line_reader_wq, &line_rq_ws->ws);
 
-	sec_left -= nr_ppas;
+	sec_left -= nr_secs;
 	if (sec_left > 0)
 		goto next_rq;
 
 out:
 	pblk_mfree(emeta_buf, l_mg->emeta_alloc_type);
 	mempool_free(line_ws, pblk->line_ws_pool);
-	atomic_dec(&pblk->gc.inflight_gc);
-	if (put_line)
-		kref_put(&line->ref, pblk_line_put);
+
+	kref_put(&line->ref, pblk_line_put);
+	atomic_dec(&gc->inflight_gc);
+
+	return;
+
+fail_free_gc_rq:
+	kfree(gc_rq);
+fail_free_emeta:
+	pblk_mfree(emeta_buf, l_mg->emeta_alloc_type);
+	pblk_put_line_back(pblk, line);
+	kref_put(&line->ref, pblk_line_put);
+	mempool_free(line_ws, pblk->line_ws_pool);
+	atomic_dec(&gc->inflight_gc);
+
+	pr_err("pblk: Failed to GC line %d\n", line->id);
 }
 
 static int pblk_gc_line(struct pblk *pblk, struct pblk_line *line)
 {
-	struct pblk_line_mgmt *l_mg = &pblk->l_mg;
-	struct pblk_line_meta *lm = &pblk->lm;
-	struct line_emeta *emeta_buf;
+	struct pblk_gc *gc = &pblk->gc;
 	struct pblk_line_ws *line_ws;
-	int ret;
 
-	line_ws = mempool_alloc(pblk->line_ws_pool, GFP_KERNEL);
-	emeta_buf = pblk_malloc(lm->emeta_len[0], l_mg->emeta_alloc_type,
-								GFP_KERNEL);
-	if (!emeta_buf) {
-		pr_err("pblk: cannot use GC emeta\n");
-		goto fail_free_ws;
-	}
+	pr_debug("pblk: line '%d' being reclaimed for GC\n", line->id);
 
-	ret = pblk_line_read_emeta(pblk, line, emeta_buf);
-	if (ret) {
-		pr_err("pblk: line %d read emeta failed (%d)\n", line->id, ret);
-		goto fail_free_emeta;
-	}
+	line_ws = mempool_alloc(pblk->line_ws_pool, GFP_KERNEL);
+	if (!line_ws)
+		return -ENOMEM;
 
 	line_ws->pblk = pblk;
 	line_ws->line = line;
-	line_ws->priv = emeta_buf;
 
-	INIT_WORK(&line_ws->ws, pblk_gc_line_ws);
-	queue_work(pblk->gc.gc_reader_wq, &line_ws->ws);
+	INIT_WORK(&line_ws->ws, pblk_gc_line_prepare_ws);
+	queue_work(gc->gc_reader_wq, &line_ws->ws);
 
 	return 0;
-
-fail_free_emeta:
-	pblk_mfree(emeta_buf, l_mg->emeta_alloc_type);
-fail_free_ws:
-	mempool_free(line_ws, pblk->line_ws_pool);
-	pblk_put_line_back(pblk, line);
-
-	return 1;
 }
 
-static void pblk_gc_lines(struct pblk *pblk, struct list_head *gc_list)
+static int pblk_gc_read(struct pblk *pblk)
 {
-	struct pblk_line *line, *tline;
+	struct pblk_gc *gc = &pblk->gc;
+	struct pblk_line *line;
 
-	list_for_each_entry_safe(line, tline, gc_list, list) {
-		if (pblk_gc_line(pblk, line))
-			pr_err("pblk: failed to GC line %d\n", line->id);
-		list_del(&line->list);
+	spin_lock(&gc->r_lock);
+	if (list_empty(&gc->r_list)) {
+		spin_unlock(&gc->r_lock);
+		return 1;
 	}
+
+	line = list_first_entry(&gc->r_list, struct pblk_line, list);
+	list_del(&line->list);
+	spin_unlock(&gc->r_lock);
+
+	pblk_gc_kick(pblk);
+
+	if (pblk_gc_line(pblk, line))
+		pr_err("pblk: failed to GC line %d\n", line->id);
+
+	return 0;
+}
+
+static void pblk_gc_reader_kick(struct pblk_gc *gc)
+{
+	wake_up_process(gc->gc_reader_ts);
 }
 
 static struct pblk_line *pblk_gc_get_victim_line(struct pblk *pblk,
@@ -301,6 +324,17 @@ static struct pblk_line *pblk_gc_get_victim_line(struct pblk *pblk,
 	return victim;
 }
 
+static bool pblk_gc_should_run(struct pblk_gc *gc, struct pblk_rl *rl)
+{
+	unsigned int nr_blocks_free, nr_blocks_need;
+
+	nr_blocks_need = pblk_rl_high_thrs(rl);
+	nr_blocks_free = pblk_rl_nr_free_blks(rl);
+
+	/* This is not critical, no need to take lock here */
+	return ((gc->gc_active) && (nr_blocks_need > nr_blocks_free));
+}
+
 /*
  * Lines with no valid sectors will be returned to the free list immediately. If
  * GC is activated - either because the free block count is under the determined
@@ -311,71 +345,83 @@ static void pblk_gc_run(struct pblk *pblk)
 {
 	struct pblk_line_mgmt *l_mg = &pblk->l_mg;
 	struct pblk_gc *gc = &pblk->gc;
-	struct pblk_line *line, *tline;
-	unsigned int nr_blocks_free, nr_blocks_need;
+	struct pblk_line *line;
 	struct list_head *group_list;
-	int run_gc, gc_group = 0;
-	int prev_gc = 0;
-	int inflight_gc = atomic_read(&gc->inflight_gc);
-	LIST_HEAD(gc_list);
+	bool run_gc;
+	int inflight_gc, gc_group = 0, prev_group = 0;
+
+	do {
+		spin_lock(&l_mg->gc_lock);
+		if (list_empty(&l_mg->gc_full_list)) {
+			spin_unlock(&l_mg->gc_lock);
+			break;
+		}
+
+		line = list_first_entry(&l_mg->gc_full_list,
+							struct pblk_line, list);
 
-	spin_lock(&l_mg->gc_lock);
-	list_for_each_entry_safe(line, tline, &l_mg->gc_full_list, list) {
 		spin_lock(&line->lock);
 		WARN_ON(line->state != PBLK_LINESTATE_CLOSED);
 		line->state = PBLK_LINESTATE_GC;
 		spin_unlock(&line->lock);
 
 		list_del(&line->list);
+		spin_unlock(&l_mg->gc_lock);
+
 		kref_put(&line->ref, pblk_line_put);
-	}
-	spin_unlock(&l_mg->gc_lock);
+	} while (1);
 
-	nr_blocks_need = pblk_rl_gc_thrs(&pblk->rl);
-	nr_blocks_free = pblk_rl_nr_free_blks(&pblk->rl);
-	run_gc = (nr_blocks_need > nr_blocks_free || gc->gc_forced);
+	run_gc = pblk_gc_should_run(&pblk->gc, &pblk->rl);
+	if (!run_gc || (atomic_read(&gc->inflight_gc) >= PBLK_GC_L_QD))
+		return;
 
 next_gc_group:
 	group_list = l_mg->gc_lists[gc_group++];
-	spin_lock(&l_mg->gc_lock);
-	while (run_gc && !list_empty(group_list)) {
-		/* No need to queue up more GC lines than we can handle */
-		if (!run_gc || inflight_gc > gc->gc_jobs_active) {
+
+	do {
+		spin_lock(&l_mg->gc_lock);
+		if (list_empty(group_list)) {
 			spin_unlock(&l_mg->gc_lock);
-			pblk_gc_lines(pblk, &gc_list);
-			return;
+			break;
 		}
 
 		line = pblk_gc_get_victim_line(pblk, group_list);
-		nr_blocks_free += atomic_read(&line->blk_in_line);
 
 		spin_lock(&line->lock);
 		WARN_ON(line->state != PBLK_LINESTATE_CLOSED);
 		line->state = PBLK_LINESTATE_GC;
-		list_move_tail(&line->list, &gc_list);
-		atomic_inc(&gc->inflight_gc);
-		inflight_gc++;
 		spin_unlock(&line->lock);
 
-		prev_gc = 1;
-		run_gc = (nr_blocks_need > nr_blocks_free || gc->gc_forced);
-	}
-	spin_unlock(&l_mg->gc_lock);
+		list_del(&line->list);
+		spin_unlock(&l_mg->gc_lock);
+
+		spin_lock(&gc->r_lock);
+		list_add_tail(&line->list, &gc->r_list);
+		spin_unlock(&gc->r_lock);
+
+		inflight_gc = atomic_inc_return(&gc->inflight_gc);
+		pblk_gc_reader_kick(gc);
 
-	pblk_gc_lines(pblk, &gc_list);
+		prev_group = 1;
 
-	if (!prev_gc && pblk->rl.rb_state > gc_group &&
-						gc_group < PBLK_NR_GC_LISTS)
+		/* No need to queue up more GC lines than we can handle */
+		run_gc = pblk_gc_should_run(&pblk->gc, &pblk->rl);
+		if (!run_gc || inflight_gc >= PBLK_GC_L_QD)
+			break;
+	} while (1);
+
+	if (!prev_group && pblk->rl.rb_state > gc_group &&
+						gc_group < PBLK_GC_NR_LISTS)
 		goto next_gc_group;
 }
 
-
-static void pblk_gc_kick(struct pblk *pblk)
+void pblk_gc_kick(struct pblk *pblk)
 {
 	struct pblk_gc *gc = &pblk->gc;
 
 	wake_up_process(gc->gc_ts);
 	pblk_gc_writer_kick(gc);
+	pblk_gc_reader_kick(gc);
 	mod_timer(&gc->gc_timer, jiffies + msecs_to_jiffies(GC_TIME_MSECS));
 }
 
@@ -413,42 +459,34 @@ static int pblk_gc_writer_ts(void *data)
 	return 0;
 }
 
-static void pblk_gc_start(struct pblk *pblk)
+static int pblk_gc_reader_ts(void *data)
 {
-	pblk->gc.gc_active = 1;
+	struct pblk *pblk = data;
 
-	pr_debug("pblk: gc start\n");
+	while (!kthread_should_stop()) {
+		if (!pblk_gc_read(pblk))
+			continue;
+		set_current_state(TASK_INTERRUPTIBLE);
+		io_schedule();
+	}
+
+	return 0;
 }
 
-int pblk_gc_status(struct pblk *pblk)
+static void pblk_gc_start(struct pblk *pblk)
 {
-	struct pblk_gc *gc = &pblk->gc;
-	int ret;
-
-	spin_lock(&gc->lock);
-	ret = gc->gc_active;
-	spin_unlock(&gc->lock);
-
-	return ret;
+	pblk->gc.gc_active = 1;
+	pr_debug("pblk: gc start\n");
 }
 
-static void __pblk_gc_should_start(struct pblk *pblk)
+void pblk_gc_should_start(struct pblk *pblk)
 {
 	struct pblk_gc *gc = &pblk->gc;
 
-	lockdep_assert_held(&gc->lock);
-
 	if (gc->gc_enabled && !gc->gc_active)
 		pblk_gc_start(pblk);
-}
-
-void pblk_gc_should_start(struct pblk *pblk)
-{
-	struct pblk_gc *gc = &pblk->gc;
 
-	spin_lock(&gc->lock);
-	__pblk_gc_should_start(pblk);
-	spin_unlock(&gc->lock);
+	pblk_gc_kick(pblk);
 }
 
 /*
@@ -457,10 +495,7 @@ void pblk_gc_should_start(struct pblk *pblk)
  */
 static void pblk_gc_stop(struct pblk *pblk, int flush_wq)
 {
-	spin_lock(&pblk->gc.lock);
 	pblk->gc.gc_active = 0;
-	spin_unlock(&pblk->gc.lock);
-
 	pr_debug("pblk: gc stop\n");
 }
 
@@ -483,20 +518,25 @@ void pblk_gc_sysfs_state_show(struct pblk *pblk, int *gc_enabled,
 	spin_unlock(&gc->lock);
 }
 
-void pblk_gc_sysfs_force(struct pblk *pblk, int force)
+int pblk_gc_sysfs_force(struct pblk *pblk, int force)
 {
 	struct pblk_gc *gc = &pblk->gc;
-	int rsv = 0;
+
+	if (force < 0 || force > 1)
+		return -EINVAL;
 
 	spin_lock(&gc->lock);
-	if (force) {
-		gc->gc_enabled = 1;
-		rsv = 64;
-	}
-	pblk_rl_set_gc_rsc(&pblk->rl, rsv);
 	gc->gc_forced = force;
-	__pblk_gc_should_start(pblk);
+
+	if (force)
+		gc->gc_enabled = 1;
+	else
+		gc->gc_enabled = 0;
 	spin_unlock(&gc->lock);
+
+	pblk_gc_should_start(pblk);
+
+	return 0;
 }
 
 int pblk_gc_init(struct pblk *pblk)
@@ -518,30 +558,58 @@ int pblk_gc_init(struct pblk *pblk)
 		goto fail_free_main_kthread;
 	}
 
+	gc->gc_reader_ts = kthread_create(pblk_gc_reader_ts, pblk,
+							"pblk-gc-reader-ts");
+	if (IS_ERR(gc->gc_reader_ts)) {
+		pr_err("pblk: could not allocate GC reader kthread\n");
+		ret = PTR_ERR(gc->gc_reader_ts);
+		goto fail_free_writer_kthread;
+	}
+
 	setup_timer(&gc->gc_timer, pblk_gc_timer, (unsigned long)pblk);
 	mod_timer(&gc->gc_timer, jiffies + msecs_to_jiffies(GC_TIME_MSECS));
 
 	gc->gc_active = 0;
 	gc->gc_forced = 0;
 	gc->gc_enabled = 1;
-	gc->gc_jobs_active = 8;
 	gc->w_entries = 0;
 	atomic_set(&gc->inflight_gc, 0);
 
-	gc->gc_reader_wq = alloc_workqueue("pblk-gc-reader-wq",
-			WQ_MEM_RECLAIM | WQ_UNBOUND, gc->gc_jobs_active);
+	/* Workqueue that reads valid sectors from a line and submit them to the
+	 * GC writer to be recycled.
+	 */
+	gc->gc_line_reader_wq = alloc_workqueue("pblk-gc-line-reader-wq",
+			WQ_MEM_RECLAIM | WQ_UNBOUND, PBLK_GC_MAX_READERS);
+	if (!gc->gc_line_reader_wq) {
+		pr_err("pblk: could not allocate GC line reader workqueue\n");
+		ret = -ENOMEM;
+		goto fail_free_reader_kthread;
+	}
+
+	/* Workqueue that prepare lines for GC */
+	gc->gc_reader_wq = alloc_workqueue("pblk-gc-line_wq",
+					WQ_MEM_RECLAIM | WQ_UNBOUND, 1);
 	if (!gc->gc_reader_wq) {
 		pr_err("pblk: could not allocate GC reader workqueue\n");
 		ret = -ENOMEM;
-		goto fail_free_writer_kthread;
+		goto fail_free_reader_line_wq;
 	}
 
 	spin_lock_init(&gc->lock);
 	spin_lock_init(&gc->w_lock);
+	spin_lock_init(&gc->r_lock);
+
+	sema_init(&gc->gc_sem, 128);
+
 	INIT_LIST_HEAD(&gc->w_list);
+	INIT_LIST_HEAD(&gc->r_list);
 
 	return 0;
 
+fail_free_reader_line_wq:
+	destroy_workqueue(gc->gc_line_reader_wq);
+fail_free_reader_kthread:
+	kthread_stop(gc->gc_reader_ts);
 fail_free_writer_kthread:
 	kthread_stop(gc->gc_writer_ts);
 fail_free_main_kthread:
@@ -555,6 +623,7 @@ void pblk_gc_exit(struct pblk *pblk)
 	struct pblk_gc *gc = &pblk->gc;
 
 	flush_workqueue(gc->gc_reader_wq);
+	flush_workqueue(gc->gc_line_reader_wq);
 
 	del_timer(&gc->gc_timer);
 	pblk_gc_stop(pblk, 1);
@@ -562,9 +631,15 @@ void pblk_gc_exit(struct pblk *pblk)
 	if (gc->gc_ts)
 		kthread_stop(gc->gc_ts);
 
-	if (pblk->gc.gc_reader_wq)
-		destroy_workqueue(pblk->gc.gc_reader_wq);
+	if (gc->gc_reader_wq)
+		destroy_workqueue(gc->gc_reader_wq);
+
+	if (gc->gc_line_reader_wq)
+		destroy_workqueue(gc->gc_line_reader_wq);
 
 	if (gc->gc_writer_ts)
 		kthread_stop(gc->gc_writer_ts);
+
+	if (gc->gc_reader_ts)
+		kthread_stop(gc->gc_reader_ts);
 }