[PATCH 1/1] bcache: process fewer btree nodes in incremental GC cycles

[Robert Pang <robertpang@google.com>]

Current incremental GC processes a minimum of 100 btree nodes per cycle,
followed by a 100ms sleep. For NVMe cache devices, where the average node
processing time is ~1ms, this leads to front-side I/O latency potentially
reaching tens or hundreds of milliseconds during GC execution.

This commit resolves this latency issue by reducing the minimum node processing
count per cycle to 10 and the inter-cycle sleep duration to 10ms. It also
integrates a check of existing GC statistics to re-scale the number of nodes
processed per sleep interval when needed, ensuring GC finishes well before the
next GC is due.

Signed-off-by: Robert Pang <robertpang@google.com>
---
 drivers/md/bcache/btree.c | 38 +++++++++++++++++---------------------
 drivers/md/bcache/util.h  |  3 +++
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index ed40d8600656..093e1edcaa53 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -88,11 +88,8 @@
  * 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 GC_MIN_NODES		10
+#define GC_SLEEP_MS		10
 
 #define PTR_DIRTY_BIT		(((uint64_t) 1 << 36))
 
@@ -1585,25 +1582,24 @@ static unsigned int btree_gc_count_keys(struct btree *b)
 
 static size_t btree_gc_min_nodes(struct cache_set *c)
 {
-	size_t min_nodes;
+	size_t min_nodes = GC_MIN_NODES;
+	uint64_t gc_max_ms = time_stat_average(&c->btree_gc_time, frequency, ms) / 2;
 
 	/*
-	 * Since incremental GC would stop 100ms 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
-	 * would run out of the buckets (since no new bucket
-	 * can be allocated during GC), and be blocked again.
-	 * So GC should not process constant nodes, but varied
-	 * nodes according to the number of btree nodes, which
-	 * 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)
+	 * The incremental garbage collector operates by processing
+	 * GC_MIN_NODES at a time, pausing for GC_SLEEP_MS between
+	 * each interval. If historical garbage collection statistics
+	 * (btree_gc_time) is available, the maximum allowable GC
+	 * duration is set to half of this observed frequency. To
+	 * prevent exceeding this maximum duration, the number of
+	 * nodes processed in the current step may be increased if
+	 * the projected completion time based on the current pace
+	 * extends beyond the allowed limit. This ensures timely GC
+	 * completion before the next GC is due.
 	 */
-	min_nodes = c->gc_stats.nodes / MAX_GC_TIMES;
-	if (min_nodes < MIN_GC_NODES)
-		min_nodes = MIN_GC_NODES;
+	if ((gc_max_ms >= GC_SLEEP_MS) &&
+	    (GC_SLEEP_MS * (c->gc_stats.nodes / min_nodes)) > gc_max_ms)
+		min_nodes = c->gc_stats.nodes / (gc_max_ms / GC_SLEEP_MS);
 
 	return min_nodes;
 }
diff --git a/drivers/md/bcache/util.h b/drivers/md/bcache/util.h
index 539454d8e2d0..21a370f444b7 100644
--- a/drivers/md/bcache/util.h
+++ b/drivers/md/bcache/util.h
@@ -305,6 +305,9 @@ static inline unsigned int local_clock_us(void)
 #define NSEC_PER_ms			NSEC_PER_MSEC
 #define NSEC_PER_sec			NSEC_PER_SEC
 
+#define time_stat_average(stats, stat, units)				\
+	div_u64((stats)->average_ ## stat >> 8, NSEC_PER_ ## units)
+
 #define __print_time_stat(stats, name, stat, units)			\
 	sysfs_print(name ## _ ## stat ## _ ## units,			\
 		    div_u64((stats)->stat >> 8, NSEC_PER_ ## units))
-- 
2.49.0.604.gff1f9ca942-goog