6.1-stable review patch. If anyone has any objections, please let me know.
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From: Yu Kuai yukuai3@huawei.com
[ Upstream commit 99dc422335d8b2bd4d105797241d3e715bae90e9 ]
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@huawei.com Link: https://lore.kernel.org/r/20240509123717.3223892-2-yukuai1@huaweicloud.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- 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 aefdf07bdc2cf..a4155f123ab38 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -933,10 +933,11 @@ 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 cc7f6a4a255c9..13a47b37acb7d 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -779,6 +779,8 @@ 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 355c4c52065b8..3afa5c8d165b1 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -986,6 +986,8 @@ 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(); } } @@ -1006,6 +1008,8 @@ 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 a186ea20f39d8..9b2f53ff4c37f 100644 --- a/block/blk.h +++ b/block/blk.h @@ -346,6 +346,7 @@ 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 ddb17c4adc8a2..f9e3ecd5ba2fa 100644 --- a/block/genhd.c +++ b/block/genhd.c @@ -123,7 +123,7 @@ 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;