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From: Yu Kuai <yukuai1@huaweicloud.com>
To: ming.lei@redhat.com,
	hch@lst.de,
	bvanassche@acm.org,
	axboe@kernel.dk
Cc: linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	yukuai3@huawei.com,
	yukuai1@huaweicloud.com,
	yi.zhang@huawei.com,
	yangerkun@huawei.com
Subject: [PATCH for-6.8/block] block: support to account io_ticks precisely
Date: Tue,  9 Jan 2024 15:13:32 +0800
Message-Id: <20240109071332.2216253-1-yukuai1@huaweicloud.com>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

From: Yu Kuai <yukuai3@huawei.com>

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.

In order to account io_ticks precisely, update_io_ticks() must know if
there are IO inflight already, and this requires overhead slightly,
hence precise io accounting is disabled by default, and user can enable
it through sysfs entry.

Noted that for rq-based devcie, part_stat_local_inc/dec() and
part_in_flight() is used to track inflight instead of iterating tags,
which is not supposed to be used in fast path because 'tags->lock' is
grabbed in blk_mq_find_and_get_req().

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
Changes from RFC v1:
 - remove the new parameter for update_io_ticks();
 - simplify update_io_ticks();
 - use swith in queue_iostats_store();
 - add missing part_stat_local_dec() in blk_account_io_merge_request();
Changes from RFC v2:
 - fix that precise is ignored for the first io in update_io_ticks();

 Documentation/ABI/stable/sysfs-block |  8 ++++--
 block/blk-core.c                     | 10 +++++--
 block/blk-merge.c                    |  3 ++
 block/blk-mq-debugfs.c               |  2 ++
 block/blk-mq.c                       | 11 +++++++-
 block/blk-sysfs.c                    | 42 ++++++++++++++++++++++++++--
 block/blk.h                          |  1 +
 block/genhd.c                        |  2 +-
 include/linux/blk-mq.h               |  1 +
 include/linux/blkdev.h               |  3 ++
 10 files changed, 74 insertions(+), 9 deletions(-)

diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
index 1fe9a553c37b..79027bf2661a 100644
--- a/Documentation/ABI/stable/sysfs-block
+++ b/Documentation/ABI/stable/sysfs-block
@@ -358,8 +358,12 @@ What:		/sys/block/<disk>/queue/iostats
 Date:		January 2009
 Contact:	linux-block@vger.kernel.org
 Description:
-		[RW] This file is used to control (on/off) the iostats
-		accounting of the disk.
+		[RW] This file is used to control the iostats accounting of the
+		disk. If this value is 0, iostats accounting is disabled; If
+		this value is 1, iostats accounting is enabled, but io_ticks is
+		accounted by sampling and the result is not accurate; If this
+		value is 2, iostats accounting is enabled and io_ticks is
+		accounted precisely, but there will be slightly more overhead.
 
 
 What:		/sys/block/<disk>/queue/logical_block_size
diff --git a/block/blk-core.c b/block/blk-core.c
index 9520ccab3050..c70dc311e3b7 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -954,11 +954,15 @@ EXPORT_SYMBOL_GPL(iocb_bio_iopoll);
 void update_io_ticks(struct block_device *part, unsigned long now, bool end)
 {
 	unsigned long stamp;
+	bool precise = blk_queue_precise_io_stat(part->bd_queue);
 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))) {
+		if (end || (precise && part_in_flight(part)))
+			__part_stat_add(part, io_ticks, now - stamp);
+		else if (!precise)
+			__part_stat_add(part, io_ticks, 1);
 	}
 	if (part->bd_partno) {
 		part = bdev_whole(part);
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 2d470cf2173e..ea5553766f90 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -779,6 +779,9 @@ 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))]);
+		if (req->rq_flags & RQF_PRECISE_IO_STAT)
+			part_stat_local_dec(req->part,
+					in_flight[op_is_write(req_op(req))]);
 		part_stat_unlock();
 	}
 }
diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 5cbeb9344f2f..4d745fc07a8a 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -86,6 +86,7 @@ static const char *const blk_queue_flag_name[] = {
 	QUEUE_FLAG_NAME(FAIL_IO),
 	QUEUE_FLAG_NAME(NONROT),
 	QUEUE_FLAG_NAME(IO_STAT),
+	QUEUE_FLAG_NAME(PRECISE_IO_STAT),
 	QUEUE_FLAG_NAME(NOXMERGES),
 	QUEUE_FLAG_NAME(ADD_RANDOM),
 	QUEUE_FLAG_NAME(SYNCHRONOUS),
@@ -252,6 +253,7 @@ static const char *const rqf_name[] = {
 	RQF_NAME(FAILED),
 	RQF_NAME(QUIET),
 	RQF_NAME(IO_STAT),
+	RQF_NAME(PRECISE_IO_STAT),
 	RQF_NAME(PM),
 	RQF_NAME(HASHED),
 	RQF_NAME(STATS),
diff --git a/block/blk-mq.c b/block/blk-mq.c
index fb29ff5cc281..5b82bafbb740 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -360,8 +360,11 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 
 	if (data->flags & BLK_MQ_REQ_PM)
 		data->rq_flags |= RQF_PM;
-	if (blk_queue_io_stat(q))
+	if (blk_queue_io_stat(q)) {
 		data->rq_flags |= RQF_IO_STAT;
+		if (blk_queue_precise_io_stat(q))
+			data->rq_flags |= RQF_PRECISE_IO_STAT;
+	}
 	rq->rq_flags = data->rq_flags;
 
 	if (data->rq_flags & RQF_SCHED_TAGS) {
@@ -999,6 +1002,9 @@ 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);
+		if (req->rq_flags & RQF_PRECISE_IO_STAT)
+			part_stat_local_dec(req->part,
+					in_flight[op_is_write(req_op(req))]);
 		part_stat_unlock();
 	}
 }
@@ -1021,6 +1027,9 @@ static inline void blk_account_io_start(struct request *req)
 
 		part_stat_lock();
 		update_io_ticks(req->part, jiffies, false);
+		if (req->rq_flags & RQF_PRECISE_IO_STAT)
+			part_stat_local_inc(req->part,
+					in_flight[op_is_write(req_op(req))]);
 		part_stat_unlock();
 	}
 }
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 40bab5975c56..3753fc28d467 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -303,7 +303,6 @@ queue_##name##_store(struct request_queue *q, const char *page, size_t count) \
 
 QUEUE_SYSFS_BIT_FNS(nonrot, NONROT, 1);
 QUEUE_SYSFS_BIT_FNS(random, ADD_RANDOM, 0);
-QUEUE_SYSFS_BIT_FNS(iostats, IO_STAT, 0);
 QUEUE_SYSFS_BIT_FNS(stable_writes, STABLE_WRITES, 0);
 #undef QUEUE_SYSFS_BIT_FNS
 
@@ -468,6 +467,45 @@ static ssize_t queue_dax_show(struct request_queue *q, char *page)
 	return queue_var_show(blk_queue_dax(q), page);
 }
 
+static ssize_t queue_iostats_show(struct request_queue *q, char *page)
+{
+	int val = 0;
+
+	if (blk_queue_io_stat(q))
+		val = blk_queue_precise_io_stat(q) ? 2 : 1;
+
+	return queue_var_show(val, page);
+}
+
+static ssize_t
+queue_iostats_store(struct request_queue *q, const char *page, size_t count)
+{
+	unsigned long nr;
+	int ret = queue_var_store(&nr, page, count);
+
+	if (ret < 0)
+		return ret;
+
+	switch (nr) {
+	case 0:
+		blk_queue_flag_clear(QUEUE_FLAG_IO_STAT, q);
+		blk_queue_flag_clear(QUEUE_FLAG_PRECISE_IO_STAT, q);
+		break;
+	case 1:
+		blk_queue_flag_set(QUEUE_FLAG_IO_STAT, q);
+		blk_queue_flag_clear(QUEUE_FLAG_PRECISE_IO_STAT, q);
+		break;
+	case 2:
+		blk_queue_flag_set(QUEUE_FLAG_IO_STAT, q);
+		blk_queue_flag_set(QUEUE_FLAG_PRECISE_IO_STAT, q);
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return count;
+}
+
 #define QUEUE_RO_ENTRY(_prefix, _name)			\
 static struct queue_sysfs_entry _prefix##_entry = {	\
 	.attr	= { .name = _name, .mode = 0444 },	\
@@ -489,6 +527,7 @@ QUEUE_RO_ENTRY(queue_max_segments, "max_segments");
 QUEUE_RO_ENTRY(queue_max_integrity_segments, "max_integrity_segments");
 QUEUE_RO_ENTRY(queue_max_segment_size, "max_segment_size");
 QUEUE_RW_ENTRY(elv_iosched, "scheduler");
+QUEUE_RW_ENTRY(queue_iostats, "iostats");
 
 QUEUE_RO_ENTRY(queue_logical_block_size, "logical_block_size");
 QUEUE_RO_ENTRY(queue_physical_block_size, "physical_block_size");
@@ -534,7 +573,6 @@ static struct queue_sysfs_entry queue_hw_sector_size_entry = {
 };
 
 QUEUE_RW_ENTRY(queue_nonrot, "rotational");
-QUEUE_RW_ENTRY(queue_iostats, "iostats");
 QUEUE_RW_ENTRY(queue_random, "add_random");
 QUEUE_RW_ENTRY(queue_stable_writes, "stable_writes");
 
diff --git a/block/blk.h b/block/blk.h
index 1ef920f72e0f..1154e87a4022 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -344,6 +344,7 @@ 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 d74fb5b4ae68..92d03cd24c79 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;
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index a676e116085f..bd925421bc4a 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -44,6 +44,7 @@ typedef __u32 __bitwise req_flags_t;
 #define RQF_QUIET		((__force req_flags_t)(1 << 11))
 /* account into disk and partition IO statistics */
 #define RQF_IO_STAT		((__force req_flags_t)(1 << 13))
+#define RQF_PRECISE_IO_STAT	((__force req_flags_t)(1 << 14))
 /* runtime pm request */
 #define RQF_PM			((__force req_flags_t)(1 << 15))
 /* on IO scheduler merge hash */
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index e1e705aef51e..81c9784baff2 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -516,6 +516,7 @@ struct request_queue {
 #define QUEUE_FLAG_NONROT	6	/* non-rotational device (SSD) */
 #define QUEUE_FLAG_VIRT		QUEUE_FLAG_NONROT /* paravirt device */
 #define QUEUE_FLAG_IO_STAT	7	/* do disk/partitions IO accounting */
+#define QUEUE_FLAG_PRECISE_IO_STAT 8	/* do disk/partitions IO accounting precisely */
 #define QUEUE_FLAG_NOXMERGES	9	/* No extended merges */
 #define QUEUE_FLAG_ADD_RANDOM	10	/* Contributes to random pool */
 #define QUEUE_FLAG_SYNCHRONOUS	11	/* always completes in submit context */
@@ -556,6 +557,8 @@ bool blk_queue_flag_test_and_set(unsigned int flag, struct request_queue *q);
 #define blk_queue_stable_writes(q) \
 	test_bit(QUEUE_FLAG_STABLE_WRITES, &(q)->queue_flags)
 #define blk_queue_io_stat(q)	test_bit(QUEUE_FLAG_IO_STAT, &(q)->queue_flags)
+#define blk_queue_precise_io_stat(q) \
+	test_bit(QUEUE_FLAG_PRECISE_IO_STAT, &(q)->queue_flags)
 #define blk_queue_add_random(q)	test_bit(QUEUE_FLAG_ADD_RANDOM, &(q)->queue_flags)
 #define blk_queue_zone_resetall(q)	\
 	test_bit(QUEUE_FLAG_ZONE_RESETALL, &(q)->queue_flags)
-- 
2.39.2