net: dqs: make struct dql more cache efficient

With the previous change, struct dqs->stall_thrs will be in the hot path
(at queue side), even if DQS is disabled.

The other fields accessed in this function (last_obj_cnt and num_queued)
are in the first cache line, let's move this field  (stall_thrs) to the
very first cache line, since there is a hole there.

This does not change the structure size, since it moves an short (2
bytes) to 4-bytes whole in the first cache line.

This is the new structure format now:

struct dql {
	unsigned int    num_queued;
	unsigned int    last_obj_cnt;
...
	short unsigned int    stall_thrs;
	/* XXX 2 bytes hole, try to pack */
...
	/* --- cacheline 1 boundary (64 bytes) --- */
...
 	/* Longest stall detected, reported to user */
	short unsigned int         stall_max;
	/* XXX 2 bytes hole, try to pack */
};

Also, read the stall_thrs (now in the very first cache line) earlier,
together with dql->num_queued (also in the first cache line).

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20240411192241.2498631-5-leitao@debian.org
Signed-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) */