net: dql: Optimize stall information population

When Dynamic Queue Limit (DQL) is set, it always populate stall
information through dql_queue_stall().  However, this information is
only necessary if a stall threshold is set, stored in struct
dql->stall_thrs.

dql_queue_stall() is cheap, but not free, since it does have memory
barriers and so forth.

Do not call dql_queue_stall() if there is no stall threshold set, and
save some CPU cycles.
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20240411192241.2498631-4-leitao@debian.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/dynamic_queue_limits.h

@@ -137,6 +137,8 @@ static inline void dql_queued(struct dql *dql, unsigned int count)
 
 	dql->num_queued += count;
 
+	/* Only populate stall information if the threshold is set */
+	if (READ_ONCE(dql->stall_thrs))
 		dql_queue_stall(dql);
 }