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net: dqs: make struct dql more cache efficient

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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>
This commit is contained in:
Breno Leitao 2024-04-11 12:22:32 -07:00 committed by Jakub Kicinski
parent 721f076b62
commit 4ba67ef3a1
2 changed files with 12 additions and 6 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

5
include/linux/dynamic_queue_limits.h
View file

@ -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) */

13
lib/dynamic_queue_limits.c
View file

@ -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);