From: Yu Kuai <yukuai3@xxxxxxxxxx>
Currently, io_ticks is accounted based on sampling, specifically
update_io_ticks() will always account io_ticks by 1 jiffies from
bdev_start_io_acct()/blk_account_io_start(), and the result can be
inaccurate, for example(HZ is 250):
Test script:
fio -filename=/dev/sda -bs=4k -rw=write -direct=1 -name=test -thinktime=4ms
Test result: util is about 90%, while the disk is really idle.
This behaviour is introduced by commit 5b18b5a73760 ("block: delete
part_round_stats and switch to less precise counting"), however, there
was a key point that is missed that this patch also improve performance
a lot:
Before the commit:
part_round_stats:
if (part->stamp != now)
stats |= 1;
part_in_flight()
-> there can be lots of task here in 1 jiffies.
part_round_stats_single()
__part_stat_add()
part->stamp = now;
After the commit:
update_io_ticks:
stamp = part->bd_stamp;
if (time_after(now, stamp))
if (try_cmpxchg())
__part_stat_add()
-> only one task can reach here in 1 jiffies.
Hence in order to account io_ticks precisely, we only need to know if
there are IO inflight at most once in one jiffies. Noted that for
rq-based device, iterating tags should not be used here because
'tags->lock' is grabbed in blk_mq_find_and_get_req(), hence
part_stat_lock_inc/dec() and part_in_flight() is used to trace inflight.
The additional overhead is quite little:
- per cpu add/dec for each IO for rq-based device;
- per cpu sum for each jiffies;
And it's verified by null-blk that there are no performance degration
under heavy IO pressure.
Fixes: 5b18b5a73760 ("block: delete part_round_stats and switch to less precise counting")
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
block/blk-core.c | 9 +++++----
block/blk-merge.c | 2 ++
block/blk-mq.c | 4 ++++
block/blk.h | 1 +
block/genhd.c | 2 +-
5 files changed, 13 insertions(+), 5 deletions(-)
diff --git a/block/blk-core.c b/block/blk-core.c
index 47400a4fe851..9ead80d6c6f0 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -978,10 +978,11 @@ void update_io_ticks(struct block_device *part, unsigned long now, bool end)
unsigned long stamp;
again:
stamp = READ_ONCE(part->bd_stamp);
- if (unlikely(time_after(now, stamp))) {
- if (likely(try_cmpxchg(&part->bd_stamp, &stamp, now)))
- __part_stat_add(part, io_ticks, end ? now - stamp : 1);
- }
+ if (unlikely(time_after(now, stamp)) &&
+ likely(try_cmpxchg(&part->bd_stamp, &stamp, now)) &&
+ (end || part_in_flight(part)))
+ __part_stat_add(part, io_ticks, now - stamp);
+
if (part->bd_partno) {
part = bdev_whole(part);
goto again;
diff --git a/block/blk-merge.c b/block/blk-merge.c
index f64115d72f3d..8534c35e0497 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -780,6 +780,8 @@ static void blk_account_io_merge_request(struct request *req)
if (blk_do_io_stat(req)) {
part_stat_lock();
part_stat_inc(req->part, merges[op_stat_group(req_op(req))]);
+ part_stat_local_dec(req->part,
+ in_flight[op_is_write(req_op(req))]);
part_stat_unlock();
}
}
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9f677ea85a52..8e01e4b32e10 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -996,6 +996,8 @@ static inline void blk_account_io_done(struct request *req, u64 now)
update_io_ticks(req->part, jiffies, true);
part_stat_inc(req->part, ios[sgrp]);
part_stat_add(req->part, nsecs[sgrp], now - req->start_time_ns);
+ part_stat_local_dec(req->part,
+ in_flight[op_is_write(req_op(req))]);
part_stat_unlock();
}
}
@@ -1018,6 +1020,8 @@ static inline void blk_account_io_start(struct request *req)
part_stat_lock();
update_io_ticks(req->part, jiffies, false);
+ part_stat_local_inc(req->part,
+ in_flight[op_is_write(req_op(req))]);
part_stat_unlock();
}
}
diff --git a/block/blk.h b/block/blk.h
index 0e46c5d30d5a..6e94c10af798 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -366,6 +366,7 @@ static inline bool blk_do_io_stat(struct request *rq)
}
void update_io_ticks(struct block_device *part, unsigned long now, bool end);
+unsigned int part_in_flight(struct block_device *part);
static inline void req_set_nomerge(struct request_queue *q, struct request *req)
{
diff --git a/block/genhd.c b/block/genhd.c
index dec2ee338fb4..8f1163d2d171 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -118,7 +118,7 @@ static void part_stat_read_all(struct block_device *part,
}
}
-static unsigned int part_in_flight(struct block_device *part)
+unsigned int part_in_flight(struct block_device *part)
{
unsigned int inflight = 0;
int cpu;
--
2.39.2
