[PATCH] blk: optimize blk_trace macros for cgroup case

[Dmitry Monakhov]

Usually blk_trace is not active, but cfq_log_xxx macros unconditionally
prepare cgroup path via blkg_path() which is suboptimal. This provoke
significant performance overhead

## Test
#modprobe null_blk queue_mode=1
#echo cfq > /sys/block/nullb0/queue/scheduler
#fio --ioengine=libaio --direct=1 --group_reporting --filename=/dev/nullb0 \
     --rw=write --iodepth=64 --bs=4k --numjobs=8 --size=4G
|         | baseline | w/ patch | gain |
| WR iops |   161483 |   189989 | +17% |
| stddev  |    14.36 |    13.19 |      |

Signed-off-by: Dmitry Monakhov <dmonakhov@xxxxxxxxxx>
---
 block/blk-throttle.c         |   17 ++++++++---------
 block/cfq-iosched.c          |   22 +++++++++++++---------
 include/linux/blktrace_api.h |    5 +++++
 3 files changed, 26 insertions(+), 18 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 5b9c6d5..6ce9750 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -238,21 +238,20 @@ static struct throtl_data *sq_to_td(struct throtl_service_queue *sq)
  * The messages are prefixed with "throtl BLKG_NAME" if @sq belongs to a
  * throtl_grp; otherwise, just "throtl".
  *
- * TODO: this should be made a function and name formatting should happen
- * after testing whether blktrace is enabled.
  */
 #define throtl_log(sq, fmt, args...)	do {				\
 	struct throtl_grp *__tg = sq_to_tg((sq));			\
 	struct throtl_data *__td = sq_to_td((sq));			\
 									\
-	(void)__td;							\
-	if ((__tg)) {							\
-		char __pbuf[128];					\
+	if (is_blk_trace_active(__td->queue)) {				\
+		if ((__tg)) {						\
+			char __pbuf[128];				\
 									\
-		blkg_path(tg_to_blkg(__tg), __pbuf, sizeof(__pbuf));	\
-		blk_add_trace_msg(__td->queue, "throtl %s " fmt, __pbuf, ##args); \
-	} else {							\
-		blk_add_trace_msg(__td->queue, "throtl " fmt, ##args);	\
+			blkg_path(tg_to_blkg(__tg), __pbuf, sizeof(__pbuf)); \
+			blk_add_trace_msg(__td->queue, "throtl %s " fmt, __pbuf, ##args); \
+		} else {						\
+			blk_add_trace_msg(__td->queue, "throtl " fmt, ##args); \
+		}							\
 	}								\
 } while (0)
 
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 5da8e6e..7b90897 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -625,20 +625,24 @@ static inline void cfqg_put(struct cfq_group *cfqg)
 }
 
 #define cfq_log_cfqq(cfqd, cfqq, fmt, args...)	do {			\
-	char __pbuf[128];						\
+	if (is_blk_trace_active((cfqd)->queue)) {			\
+		char __pbuf[128];					\
 									\
-	blkg_path(cfqg_to_blkg((cfqq)->cfqg), __pbuf, sizeof(__pbuf));	\
-	blk_add_trace_msg((cfqd)->queue, "cfq%d%c%c %s " fmt, (cfqq)->pid, \
-			cfq_cfqq_sync((cfqq)) ? 'S' : 'A',		\
-			cfqq_type((cfqq)) == SYNC_NOIDLE_WORKLOAD ? 'N' : ' ',\
-			  __pbuf, ##args);				\
+		blkg_path(cfqg_to_blkg((cfqq)->cfqg), __pbuf, sizeof(__pbuf)); \
+		blk_add_trace_msg((cfqd)->queue, "cfq%d%c%c %s " fmt, (cfqq)->pid, \
+				  cfq_cfqq_sync((cfqq)) ? 'S' : 'A',	\
+				  cfqq_type((cfqq)) == SYNC_NOIDLE_WORKLOAD ? 'N' : ' ', \
+				  __pbuf, ##args);			\
+	}								\
 } while (0)
 
 #define cfq_log_cfqg(cfqd, cfqg, fmt, args...)	do {			\
-	char __pbuf[128];						\
+	if (is_blk_trace_active((cfqd)->queue)) {			\
+		char __pbuf[128];					\
 									\
-	blkg_path(cfqg_to_blkg(cfqg), __pbuf, sizeof(__pbuf));		\
-	blk_add_trace_msg((cfqd)->queue, "%s " fmt, __pbuf, ##args);	\
+		blkg_path(cfqg_to_blkg(cfqg), __pbuf, sizeof(__pbuf));	\
+		blk_add_trace_msg((cfqd)->queue, "%s " fmt, __pbuf, ##args); \
+	}								\
 } while (0)
 
 static inline void cfqg_stats_update_io_add(struct cfq_group *cfqg,
diff --git a/include/linux/blktrace_api.h b/include/linux/blktrace_api.h
index afc1343..2981c9e 100644
--- a/include/linux/blktrace_api.h
+++ b/include/linux/blktrace_api.h
@@ -28,6 +28,11 @@ struct blk_trace {
 	atomic_t dropped;
 };
 
+static inline bool is_blk_trace_active(struct request_queue *q)
+{
+	return q->blk_trace;
+}
+
 extern int blk_trace_ioctl(struct block_device *, unsigned, char __user *);
 extern void blk_trace_shutdown(struct request_queue *);
 extern int do_blk_trace_setup(struct request_queue *q, char *name,
-- 
1.7.1

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