bcache: dynamic incremental gc

During GC, IO performance would be reduced by half or more.
According to our test data, when nvme is used as the cache,
it takes about 1ms for GC to handle each node (block 4k and
bucket 512k).

So, GC process at least 100 nodes each time, resulting in
IOPS decreasing by half and latency increasing.

This patch add some cost statistics and hold the inflight peak.
When IO depth up to maximum, gc sleep and handle these requests.
GC sleep time dynamically calculate based on gc_cost.

Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>

drivers/md/bcache/bcache.h

@@ -471,6 +471,14 @@ struct cache {
 };
 
 struct gc_stat {
+	uint64_t		gc_cost;
+	uint64_t		sleep_cost;
+	uint64_t		average_cost;
+	uint64_t		start_time;
+	uint64_t		finish_time;
+	size_t			max_inflight;
+
+	size_t			times;
 	size_t			nodes;
 	size_t			nodes_pre;
 	size_t			key_bytes;

drivers/md/bcache/btree.c

@@ -88,11 +88,12 @@
  * Test module load/unload
  */
 
-#define MAX_NEED_GC		64
-#define MAX_SAVE_PRIO		72
 #define MAX_GC_TIMES		100
 #define MIN_GC_NODES		100
-#define GC_SLEEP_MS		100
+#define MAX_GC_NODES		1000
+#define MAX_GC_PERCENT		10
+#define MIN_GC_SLEEP_MS		10
+#define MAX_GC_SLEEP_MS		1000
 
 #define PTR_DIRTY_BIT		(((uint64_t) 1 << 36))
 
@@ -1542,12 +1543,56 @@ static unsigned int btree_gc_count_keys(struct btree *b)
 	return ret;
 }
 
-static size_t btree_gc_min_nodes(struct cache_set *c)
+static uint64_t btree_gc_sleep_ms(struct cache_set *c, struct gc_stat *gc)
+{
+	uint64_t now = local_clock();
+	uint64_t expect_time, sleep_time = 0;
+
+	/*
+	 * GC maybe process very few nodes when IO requests are very frequent.
+	 * If the sleep time is constant (100ms) each time, whole GC would last
+	 * a long time.
+	 * The IO performance also decline if a single GC takes a long time
+	 * (such as single GC 100ms and sleep 100ms, IOPS is only half).
+	 * So GC sleep time should be calculated dynamically based on gc_cost.
+	 */
+	gc->finish_time = time_after64(now, gc->start_time)
+				? now - gc->start_time : 0;
+	gc->gc_cost = gc->finish_time > gc->sleep_cost
+			? gc->finish_time - gc->sleep_cost : 0;
+	expect_time = div_u64(gc->gc_cost * (100 - MAX_GC_PERCENT), MAX_GC_PERCENT);
+	if (expect_time > gc->sleep_cost)
+		sleep_time = div_u64(expect_time - gc->sleep_cost, NSEC_PER_MSEC);
+
+	if (sleep_time < MIN_GC_SLEEP_MS)
+		sleep_time = MIN_GC_SLEEP_MS;
+	if (sleep_time > MAX_GC_SLEEP_MS)
+		sleep_time = MAX_GC_SLEEP_MS;
+
+	return sleep_time;
+}
+
+static size_t btree_gc_min_nodes(struct cache_set *c, struct gc_stat *gc)
 {
 	size_t min_nodes;
+	size_t inflight;
+
+	/*
+	 * If there are no requests, the GC is not stopped. Also, we hope to
+	 * process the increasing number of IO requests immediately and hold
+	 * the inflight peak. When IO depth up to maximum, gc sleep and handle
+	 * these requests.
+	 */
+	inflight = atomic_read(&c->search_inflight);
+	if (inflight <= 0)
+		return max(c->gc_stats.nodes, gc->nodes) + 1;
+	if (inflight > gc->max_inflight)
+		gc->max_inflight = inflight;
+	if (inflight >= gc->max_inflight)
+		return 1;
 
 	/*
-	 * Since incremental GC would stop 100ms when front
+	 * Since incremental GC would dynamic sleep when front
 	 * side I/O comes, so when there are many btree nodes,
 	 * if GC only processes constant (100) nodes each time,
 	 * GC would last a long time, and the front side I/Os
@@ -1558,11 +1603,14 @@ static size_t btree_gc_min_nodes(struct cache_set *c)
 	 * realized by dividing GC into constant(100) times,
 	 * so when there are many btree nodes, GC can process
 	 * more nodes each time, otherwise, GC will process less
-	 * nodes each time (but no less than MIN_GC_NODES)
+	 * nodes each time (but no less than MIN_GC_NODES and
+	 * no more than MAX_GC_NODES)
 	 */
 	min_nodes = c->gc_stats.nodes / MAX_GC_TIMES;
 	if (min_nodes < MIN_GC_NODES)
 		min_nodes = MIN_GC_NODES;
+	if (min_nodes > MAX_GC_NODES)
+		min_nodes = MAX_GC_NODES;
 
 	return min_nodes;
 }
@@ -1633,8 +1681,7 @@ static int btree_gc_recurse(struct btree *b, struct btree_op *op,
 		memmove(r + 1, r, sizeof(r[0]) * (GC_MERGE_NODES - 1));
 		r->b = NULL;
 
-		if (atomic_read(&b->c->search_inflight) &&
-		    gc->nodes >= gc->nodes_pre + btree_gc_min_nodes(b->c)) {
+		if (gc->nodes >= gc->nodes_pre + btree_gc_min_nodes(b->c, gc)) {
 			gc->nodes_pre =  gc->nodes;
 			ret = -EAGAIN;
 			break;
@@ -1789,7 +1836,7 @@ static void bch_btree_gc(struct cache_set *c)
 	struct gc_stat stats;
 	struct closure writes;
 	struct btree_op op;
-	uint64_t start_time = local_clock();
+	uint64_t sleep_time;
 
 	trace_bcache_gc_start(c);
 
@@ -1798,24 +1845,34 @@ static void bch_btree_gc(struct cache_set *c)
 	bch_btree_op_init(&op, SHRT_MAX);
 
 	btree_gc_start(c);
+	stats.start_time = local_clock();
 
 	/* if CACHE_SET_IO_DISABLE set, gc thread should stop too */
 	do {
+		stats.times++;
 		ret = bcache_btree_root(gc_root, c, &op, &writes, &stats);
 		closure_sync(&writes);
 		cond_resched();
 
-		if (ret == -EAGAIN)
+		sleep_time = btree_gc_sleep_ms(c, &stats);
+		if (ret == -EAGAIN) {
+			stats.sleep_cost += sleep_time * NSEC_PER_MSEC;
 			schedule_timeout_interruptible(msecs_to_jiffies
-						       (GC_SLEEP_MS));
-		else if (ret)
+						       (sleep_time));
+		} else if (ret)
 			pr_warn("gc failed!\n");
 	} while (ret && !test_bit(CACHE_SET_IO_DISABLE, &c->flags));
 
 	bch_btree_gc_finish(c);
 	wake_up_allocators(c);
 
-	bch_time_stats_update(&c->btree_gc_time, start_time);
+	bch_time_stats_update(&c->btree_gc_time, stats.start_time);
+	stats.average_cost = stats.gc_cost / stats.nodes;
+	pr_info("gc %llu times with %llu nodes, sleep %llums, "
+		"average gc cost %lluus per node",
+		(uint64_t)stats.times, (uint64_t)stats.nodes,
+		div_u64(stats.sleep_cost, NSEC_PER_MSEC),
+		div_u64(stats.average_cost, NSEC_PER_USEC));
 
 	stats.key_bytes *= sizeof(uint64_t);
 	stats.data	<<= 9;