Merge branch 'net-dqs-optimize-if-stall-threshold-is-not-set'

Breno Leitao says:

====================
net: dqs: optimize if stall threshold is not set

Here are four patches aimed at enhancing the Dynamic Queue Limit (DQL)
subsystem within the networking stack.

The first two commits involve code refactoring, while the third patch
introduces the actual change. The fourth patch just improves the cache
locality.

Typically, when DQL is enabled, stall information is always populated
through dql_queue_stall(). However, this information is only necessary
if a stall threshold is set, which is stored in struct dql->stall_thrs.

Although dql_queue_stall() is relatively inexpensive, it is not entirely
free due to memory barriers and similar overheads.

To optimize performance, refrain from calling dql_queue_stall() when no
stall threshold is set, thus avoiding the processing of unnecessary
information.

v1: https://lore.kernel.org/all/20240404145939.3601097-1-leitao@debian.org/
====================

Link: https://lore.kernel.org/r/20240411192241.2498631-1-leitao@debian.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/dynamic_queue_limits.h

@@ -50,6 +50,9 @@ struct dql {
 	unsigned int	adj_limit;		/* limit + num_completed */
 	unsigned int	last_obj_cnt;		/* Count at last queuing */
 
+	/* Stall threshold (in jiffies), defined by user */
+	unsigned short	stall_thrs;
+
 	unsigned long	history_head;		/* top 58 bits of jiffies */
 	/* stall entries, a bit per entry */
 	unsigned long	history[DQL_HIST_LEN];
@@ -71,8 +74,6 @@ struct dql {
 	unsigned int	min_limit;		/* Minimum limit */
 	unsigned int	slack_hold_time;	/* Time to measure slack */
 
-	/* Stall threshold (in jiffies), defined by user */
-	unsigned short	stall_thrs;
 	/* Longest stall detected, reported to user */
 	unsigned short	stall_max;
 	unsigned long	last_reap;		/* Last reap (in jiffies) */
@@ -83,27 +84,11 @@ struct dql {
 #define DQL_MAX_OBJECT (UINT_MAX / 16)
 #define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
 
-/*
- * Record number of objects queued. Assumes that caller has already checked
- * availability in the queue with dql_avail.
- */
-static inline void dql_queued(struct dql *dql, unsigned int count)
+/* Populate the bitmap to be processed later in dql_check_stall() */
+static inline void dql_queue_stall(struct dql *dql)
 {
 	unsigned long map, now, now_hi, i;
 
-	BUG_ON(count > DQL_MAX_OBJECT);
-
-	dql->last_obj_cnt = count;
-
-	/* We want to force a write first, so that cpu do not attempt
-	 * to get cache line containing last_obj_cnt, num_queued, adj_limit
-	 * in Shared state, but directly does a Request For Ownership
-	 * It is only a hint, we use barrier() only.
-	 */
-	barrier();
-
-	dql->num_queued += count;
-
 	now = jiffies;
 	now_hi = now / BITS_PER_LONG;
 
@@ -133,6 +118,31 @@ static inline void dql_queued(struct dql *dql, unsigned int count)
 		WRITE_ONCE(DQL_HIST_ENT(dql, now_hi), map | BIT_MASK(now));
 }
 
+/*
+ * Record number of objects queued. Assumes that caller has already checked
+ * availability in the queue with dql_avail.
+ */
+static inline void dql_queued(struct dql *dql, unsigned int count)
+{
+	if (WARN_ON_ONCE(count > DQL_MAX_OBJECT))
+		return;
+
+	dql->last_obj_cnt = count;
+
+	/* We want to force a write first, so that cpu do not attempt
+	 * to get cache line containing last_obj_cnt, num_queued, adj_limit
+	 * in Shared state, but directly does a Request For Ownership
+	 * It is only a hint, we use barrier() only.
+	 */
+	barrier();
+
+	dql->num_queued += count;
+
+	/* Only populate stall information if the threshold is set */
+	if (READ_ONCE(dql->stall_thrs))
+		dql_queue_stall(dql);
+}
+
 /* Returns how many objects can be queued, < 0 indicates over limit. */
 static inline int dql_avail(const struct dql *dql)
 {

lib/dynamic_queue_limits.c

@@ -15,12 +15,10 @@
 #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
 #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
 
-static void dql_check_stall(struct dql *dql)
+static void dql_check_stall(struct dql *dql, unsigned short stall_thrs)
 {
-	unsigned short stall_thrs;
 	unsigned long now;
 
-	stall_thrs = READ_ONCE(dql->stall_thrs);
 	if (!stall_thrs)
 		return;
 
@@ -86,9 +84,16 @@ void dql_completed(struct dql *dql, unsigned int count)
 {
 	unsigned int inprogress, prev_inprogress, limit;
 	unsigned int ovlimit, completed, num_queued;
+	unsigned short stall_thrs;
 	bool all_prev_completed;
 
 	num_queued = READ_ONCE(dql->num_queued);
+	/* Read stall_thrs in advance since it belongs to the same (first)
+	 * cache line as ->num_queued. This way, dql_check_stall() does not
+	 * need to touch the first cache line again later, reducing the window
+	 * of possible false sharing.
+	 */
+	stall_thrs = READ_ONCE(dql->stall_thrs);
 
 	/* Can't complete more than what's in queue */
 	BUG_ON(count > num_queued - dql->num_completed);
@@ -178,7 +183,7 @@ void dql_completed(struct dql *dql, unsigned int count)
 	dql->num_completed = completed;
 	dql->prev_num_queued = num_queued;
 
-	dql_check_stall(dql);
+	dql_check_stall(dql, stall_thrs);
 }
 EXPORT_SYMBOL(dql_completed);