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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 RFC v2] block: support to account io_ticks precisely
Date: Wed,  3 Jan 2024 15:15:15 +0800
Message-Id: <20240103071515.2477311-1-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 in v2:
 - 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()

 Documentation/ABI/stable/sysfs-block |  8 ++++--
 block/blk-core.c                     | 10 +++++--
 block/blk-merge.c                    |  3 ++
 block/blk-mq.c                       | 12 ++++++--
 block/blk-sysfs.c                    | 42 ++++++++++++++++++++++++++--
 block/blk.h                          |  1 +
 block/genhd.c                        |  2 +-
 include/linux/blk-mq.h               |  1 +
 include/linux/blkdev.h               |  3 ++
 9 files changed, 72 insertions(+), 10 deletions(-)

diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
index 1fe9a553c37b..e5fedecf7bdf 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 overhead.
 
 
 What:		/sys/block/<disk>/queue/logical_block_size
diff --git a/block/blk-core.c b/block/blk-core.c
index 9520ccab3050..f0d347ab0f34 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -956,9 +956,13 @@ 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))) {
+		if (end || (blk_queue_precise_io_stat(part->bd_queue) &&
+			    part_in_flight(part)))
+			__part_stat_add(part, io_ticks, now - stamp);
+		else
+			__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.c b/block/blk-mq.c
index fb29ff5cc281..0cb0e8db9dfd 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();
 	}
 }
@@ -1006,7 +1012,6 @@ static inline void blk_account_io_done(struct request *req, u64 now)
 static inline void blk_account_io_start(struct request *req)
 {
 	trace_block_io_start(req);
-
 	if (blk_do_io_stat(req)) {
 		/*
 		 * All non-passthrough requests are created from a bio with one
@@ -1021,6 +1026,9 @@ 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..ce22dc29b5f1 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 sprintf(page, "%u\n", val);
+}
+
+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 94701a63ad8a..575099f28a8e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -517,6 +517,7 @@ 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 */
@@ -557,6 +558,8 @@ 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