alinux: block-throttle: only do io statistics if needed

task #29063222

Current blk throttle codes always do io statistics even though users
don't specify valid throttle rules, which will introduce significant
overheads for applications that don't use blk throttle function and
is wrose in arm, see below perf data captured in arm:

sudo taskset -c 66 fio -ioengine=io_uring -sqthread_poll=1 -hipri=1
-sqthread_poll_cpu=65 -registerfiles=1 -fixedbufs=1 -direct=1
-filename=/dev/nvme0n1 -bs=4k -iodepth=8 -rw=randwrite  -time_based
-ramp_time=30 -runtime=60  -name="test"

Samples: 25K of event 'cycles', Event count (approx.): 16586974662
Overhead  Command      Shared Object      Symbol
   3.54%  io_uring-sq  [kernel.kallsyms]  [k]
throtl_stats_update_completion
   0.89%  io_uring-sq  [kernel.kallsyms]  [k] throtl_bio_end_io
   0.66%  io_uring-sq  [kernel.kallsyms]  [k] blk_throtl_bio
   0.05%  io_uring-sq  [kernel.kallsyms]  [k] blk_throtl_stat_add
   0.05%  io_uring-sq  [kernel.kallsyms]  [k] throtl_track_latency
   0.01%  io_uring-sq  [kernel.kallsyms]  [k] blk_throtl_bio_endio

Samples: 25K of event 'cycles', Event count (approx.): 16586974662
Overhead  Command      Shared Object      Symbol
   1.62%  io_uring-sq  [kernel.kallsyms]  [k] io_submit_sqes
   1.06%  io_uring-sq  [kernel.kallsyms]  [k] io_issue_sqe
   0.32%  io_uring-sq  [kernel.kallsyms]  [k] __io_queue_sqe
   0.06%  io_uring-sq  [kernel.kallsyms]  [k] io_queue_sqe

Above test doesn't set valid blk throttle rules, but the overhead
introduced by blk throttle is even bigger than many io_uring framework
functions, which is not acceptable.

To improve this issue, only do do io statistics if users specify valid
blk throttle rules, and this will also improve performance.

Before this patch:
clat (usec): min=5, max=6871, avg=18.70, stdev=17.89
 lat (usec): min=9, max=6871, avg=18.84, stdev=17.89
WRITE: bw=1618MiB/s (1697MB/s), 1618MiB/s-1618MiB/s (1697MB/s-1697MB/s),
io=94.8GiB (102GB), run=60001-60001msec

With this patch:
clat (usec): min=5, max=7554, avg=17.49, stdev=18.24
lat (usec): min=9, max=7554, avg=17.62, stdev=18.24
 WRITE: bw=1727MiB/s (1810MB/s), 1727MiB/s-1727MiB/s
(1810MB/s-1810MB/s), io=101GiB (109GB), run=60001-60001msec

About 6.6% bps improvement and 6.4% latency reduction.
Signed-off-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

block/blk-throttle.c

@@ -2324,6 +2324,7 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 	struct throtl_service_queue *sq;
 	bool rw = bio_data_dir(bio);
 	bool throttled = false;
+	bool has_rules = tg->has_rules[rw];
 	struct throtl_data *td = tg->td;
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
@@ -2332,11 +2333,11 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 	if (bio_flagged(bio, BIO_THROTTLED))
 		goto out;
 
-	throtl_bio_stats_start(bio, tg);
-
-	if (!tg->has_rules[rw])
+	if (!has_rules)
 		goto out;
 
+	throtl_bio_stats_start(bio, tg);
+
 	spin_lock_irq(q->queue_lock);
 
 	throtl_update_latency_buckets(td);
@@ -2435,7 +2436,7 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 out_unlock:
 	spin_unlock_irq(q->queue_lock);
 out:
-	if (!throttled)
+	if (!throttled && has_rules)
 		bio_set_io_start_time_ns(bio);
 	bio_set_flag(bio, BIO_THROTTLED);