[RFC 0/1] block: export I/O latency histograms

[RFC 0/1] block: export I/O latency histograms

[Diangang Li]

From: Diangang Li <lidiangang@bytedance.com>

Hi,

The existing block I/O statistics count completed I/Os and accumulate the
time spent in each operation group. That works for average latency, but
not for the tail. Once the time is folded into a single total, userspace
cannot tell whether a device saw a steady stream of moderate I/Os or a
small number of very slow ones.

This RFC adds cumulative latency histograms for block devices and
partitions. The new accounting is in the same completion paths as the
existing I/O statistics and uses the same operation groups: read, write,
discard, and flush.

Two proc files are added:

  /proc/disk_lat_buckets
        bucket upper bounds, in microseconds

  /proc/disk_lat_hists
        cumulative histogram counters

/proc/disk_lat_hists follows the shape of /proc/diskstats. Each
reported device or partition has four consecutive lines, in read,
write, discard, flush order. Each line starts with the major number,
minor number, and device name, followed by the bucket counters.
Userspace can sample the file twice and compute interval histograms and
percentiles from the deltas.

eBPF is useful for targeted debugging, but it is not a good match for
this interface. These counters are block accounting data, tied to the
same accounting points as diskstats and readable without a resident
userspace collector.

The histogram storage is per block_device and optional. If allocation
fails, bd_lat_hist remains NULL and regular I/O statistics keep working.
The record side uses per-cpu counters.

The current bucket table has 24 upper bounds, from 10 us to 8 seconds,
which gives 25 counters. That covers both fast NVMe devices and slow
disks without making the per-device state too large.

Fio tests on NVMe and HDD devices did not show a consistent performance
regression, and confirmed that histogram deltas match the corresponding
diskstats completion counters.

Diangang Li (1):
  block: export I/O latency histograms

 Documentation/ABI/testing/procfs-diskstats |  25 ++++
 block/Makefile                             |   2 +-
 block/bdev.c                               |   2 +
 block/blk-core.c                           |   4 +-
 block/blk-flush.c                          |   5 +-
 block/blk-mq.c                             |   4 +-
 block/blk.h                                |   7 +
 block/disk-lat-hist.c                      | 158 +++++++++++++++++++++
 block/genhd.c                              |  10 ++
 include/linux/blk_types.h                  |   1 +
 10 files changed, 213 insertions(+), 5 deletions(-)
 create mode 100644 block/disk-lat-hist.c

-- 
2.39.5

[RFC 1/1] block: export I/O latency histograms

[Diangang Li]

From: Diangang Li <lidiangang@bytedance.com>

The existing block I/O statistics expose completed I/O counts and total
elapsed time for each operation group. Userspace can derive average
latency from those counters, but it cannot recover tail latency
information such as P99 from cumulative totals.

Add optional per-block-device latency histogram accounting for read,
write, discard and flush statistics groups. The counters follow the
existing I/O statistics accounting paths and are exported through
/proc/disk_lat_hists.

Add /proc/disk_lat_buckets to expose the bucket upper bounds in
microseconds so userspace can interpret each histogram counter.

Histogram storage is allocated per block_device and treated as optional.
If allocation fails, regular I/O statistics continue to work and the
histogram output skips that device.

Signed-off-by: Diangang Li <lidiangang@bytedance.com>
---
 Documentation/ABI/testing/procfs-diskstats |  25 ++++
 block/Makefile                             |   2 +-
 block/bdev.c                               |   2 +
 block/blk-core.c                           |   4 +-
 block/blk-flush.c                          |   5 +-
 block/blk-mq.c                             |   4 +-
 block/blk.h                                |   7 +
 block/disk-lat-hist.c                      | 158 +++++++++++++++++++++
 block/genhd.c                              |  10 ++
 include/linux/blk_types.h                  |   1 +
 10 files changed, 213 insertions(+), 5 deletions(-)
 create mode 100644 block/disk-lat-hist.c

diff --git a/Documentation/ABI/testing/procfs-diskstats b/Documentation/ABI/testing/procfs-diskstats
index 6a719cf2075cd..015c33f5c150b 100644
--- a/Documentation/ABI/testing/procfs-diskstats
+++ b/Documentation/ABI/testing/procfs-diskstats
@@ -41,3 +41,28 @@ Description:
 		==  =====================================
 
 		For more details refer to Documentation/admin-guide/iostats.rst
+
+What:		/proc/disk_lat_buckets
+Date:		July 2026
+Contact:	Linux block layer mailing list <linux-block@vger.kernel.org>
+Description:
+		Contains the latency histogram bucket upper bounds, in
+		microseconds. The 24 bounds define the 25 counters in
+		/proc/disk_lat_hists. The first counter covers latencies up to
+		10 us, and the last counter covers latencies above 8 seconds.
+
+What:		/proc/disk_lat_hists
+Date:		July 2026
+Contact:	Linux block layer mailing list <linux-block@vger.kernel.org>
+Description:
+		Contains cumulative I/O latency histogram counters for block
+		devices and partitions. Each reported device or partition has
+		four consecutive lines, in read, write, discard, flush order.
+		Each line has 28 fields:
+
+		==  ===================================
+		 1  major number
+		 2  minor number
+		 3  device name
+		 4-28 cumulative latency bucket counters
+		==  ===================================
diff --git a/block/Makefile b/block/Makefile
index e7bd320e3d697..a24850cf1d51f 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -11,7 +11,7 @@ obj-y		:= bdev.o fops.o bio.o elevator.o blk-core.o blk-sysfs.o \
 			blk-mq-tag.o blk-mq-dma.o blk-stat.o \
 			blk-mq-sysfs.o blk-mq-cpumap.o blk-mq-sched.o ioctl.o \
 			genhd.o ioprio.o badblocks.o partitions/ blk-rq-qos.o \
-			disk-events.o blk-ia-ranges.o early-lookup.o
+			disk-events.o blk-ia-ranges.o early-lookup.o disk-lat-hist.o
 
 obj-$(CONFIG_BLK_ERROR_INJECTION) += error-injection.o
 obj-$(CONFIG_BLK_DEV_BSG_COMMON) += bsg.o
diff --git a/block/bdev.c b/block/bdev.c
index 85ce57bd2ae4f..d389772515e4c 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -394,6 +394,7 @@ static void bdev_free_inode(struct inode *inode)
 {
 	struct block_device *bdev = I_BDEV(inode);
 
+	disk_lat_hist_free(bdev);
 	free_percpu(bdev->bd_stats);
 	kfree(bdev->bd_meta_info);
 	security_bdev_free(bdev);
@@ -483,6 +484,7 @@ struct block_device *bdev_alloc(struct gendisk *disk, u8 partno)
 		iput(inode);
 		return NULL;
 	}
+	disk_lat_hist_alloc(bdev);
 	bdev->bd_disk = disk;
 	return bdev;
 }
diff --git a/block/blk-core.c b/block/blk-core.c
index 365641266c9e8..8d9c4eb850465 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1103,12 +1103,14 @@ void bdev_end_io_acct(struct block_device *bdev, enum req_op op,
 	const int sgrp = op_stat_group(op);
 	unsigned long now = READ_ONCE(jiffies);
 	unsigned long duration = now - start_time;
+	u64 duration_ns = jiffies_to_nsecs(duration);
 
 	part_stat_lock();
 	update_io_ticks(bdev, now, true);
 	part_stat_inc(bdev, ios[sgrp]);
 	part_stat_add(bdev, sectors[sgrp], sectors);
-	part_stat_add(bdev, nsecs[sgrp], jiffies_to_nsecs(duration));
+	part_stat_add(bdev, nsecs[sgrp], duration_ns);
+	disk_lat_hist_record_part(bdev, sgrp, duration_ns);
 	bdev_dec_in_flight(bdev, op);
 	part_stat_unlock();
 }
diff --git a/block/blk-flush.c b/block/blk-flush.c
index 403a46c864117..a1fbd749b6607 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -124,11 +124,12 @@ static void blk_flush_restore_request(struct request *rq)
 static void blk_account_io_flush(struct request *rq)
 {
 	struct block_device *part = rq->q->disk->part0;
+	u64 nsecs = blk_time_get_ns() - rq->start_time_ns;
 
 	part_stat_lock();
 	part_stat_inc(part, ios[STAT_FLUSH]);
-	part_stat_add(part, nsecs[STAT_FLUSH],
-		      blk_time_get_ns() - rq->start_time_ns);
+	part_stat_add(part, nsecs[STAT_FLUSH], nsecs);
+	disk_lat_hist_record_part(part, STAT_FLUSH, nsecs);
 	part_stat_unlock();
 }
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 88cb5acc4f39e..231bd531803b4 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1077,11 +1077,13 @@ static inline void blk_account_io_done(struct request *req, u64 now)
 	 */
 	if ((req->rq_flags & (RQF_IO_STAT|RQF_FLUSH_SEQ)) == RQF_IO_STAT) {
 		const int sgrp = op_stat_group(req_op(req));
+		u64 nsecs = now - req->start_time_ns;
 
 		part_stat_lock();
 		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_add(req->part, nsecs[sgrp], nsecs);
+		disk_lat_hist_record_part(req->part, sgrp, nsecs);
 		bdev_dec_in_flight(req->part, req_op(req));
 		part_stat_unlock();
 	}
diff --git a/block/blk.h b/block/blk.h
index 25af8ac5ef0f7..c79222bd13194 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -345,6 +345,13 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
 bool blk_bio_list_merge(struct request_queue *q, struct list_head *list,
 			struct bio *bio, unsigned int nr_segs);
 
+void disk_lat_hist_alloc(struct block_device *bdev);
+void disk_lat_hist_free(struct block_device *bdev);
+void disk_lat_hist_set_all(struct block_device *bdev, int value);
+void disk_lat_hist_record_part(struct block_device *part, int sgrp, u64 nsec);
+int disk_lat_buckets_show(struct seq_file *seqf, void *v);
+int disk_lat_hists_show(struct seq_file *seqf, void *v);
+
 /*
  * Plug flush limits
  */
diff --git a/block/disk-lat-hist.c b/block/disk-lat-hist.c
new file mode 100644
index 0000000000000..1ef2b33cb68c3
--- /dev/null
+++ b/block/disk-lat-hist.c
@@ -0,0 +1,158 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/blkdev.h>
+#include <linux/part_stat.h>
+#include <linux/percpu.h>
+#include <linux/seq_file.h>
+
+#include "blk.h"
+
+#define DISK_LAT_HIST_BOUNDS	24
+#define DISK_LAT_HIST_BUCKETS	(DISK_LAT_HIST_BOUNDS + 1)
+
+struct disk_lat_hist {
+	u64 buckets[NR_STAT_GROUPS][DISK_LAT_HIST_BUCKETS];
+};
+
+static const u64 disk_lat_hist_bounds_us[DISK_LAT_HIST_BOUNDS] = {
+	10, 20, 40, 80,
+	100, 200, 400, 800,
+	1000, 2000, 4000, 8000,
+	10000, 20000, 40000, 80000,
+	100000, 200000, 400000, 800000,
+	1000000, 2000000, 4000000, 8000000,
+};
+
+static const int disk_lat_hist_order[NR_STAT_GROUPS] = {
+	STAT_READ,
+	STAT_WRITE,
+	STAT_DISCARD,
+	STAT_FLUSH,
+};
+
+void disk_lat_hist_alloc(struct block_device *bdev)
+{
+	/*
+	 * Latency histograms are optional. If allocation fails,
+	 * bd_lat_hist stays NULL; the record path skips histogram
+	 * accounting and regular I/O statistics are unaffected.
+	 */
+	bdev->bd_lat_hist = alloc_percpu(struct disk_lat_hist);
+	if (!bdev->bd_lat_hist)
+		pr_warn_once("block: failed to allocate latency histograms\n");
+}
+
+void disk_lat_hist_free(struct block_device *bdev)
+{
+	if (!bdev->bd_lat_hist)
+		return;
+	free_percpu(bdev->bd_lat_hist);
+	bdev->bd_lat_hist = NULL;
+}
+
+void disk_lat_hist_set_all(struct block_device *bdev, int value)
+{
+	int cpu;
+
+	if (!bdev->bd_lat_hist)
+		return;
+
+	for_each_possible_cpu(cpu)
+		memset(per_cpu_ptr(bdev->bd_lat_hist, cpu), value,
+		       sizeof(struct disk_lat_hist));
+}
+
+static void disk_lat_hist_record(struct block_device *bdev, int sgrp,
+				 int bucket)
+{
+	if (!bdev || !bdev->bd_lat_hist)
+		return;
+	__this_cpu_inc(bdev->bd_lat_hist->buckets[sgrp][bucket]);
+}
+
+static int disk_lat_hist_bucket(u64 nsec)
+{
+	int low = 0, high = DISK_LAT_HIST_BOUNDS;
+
+	while (low < high) {
+		int mid = low + (high - low) / 2;
+
+		if (nsec <= disk_lat_hist_bounds_us[mid] * NSEC_PER_USEC)
+			high = mid;
+		else
+			low = mid + 1;
+	}
+
+	return low;
+}
+
+void disk_lat_hist_record_part(struct block_device *part, int sgrp, u64 nsec)
+{
+	struct block_device *whole;
+	int bucket;
+
+	if (sgrp < 0 || sgrp >= NR_STAT_GROUPS || !part || !part->bd_disk)
+		return;
+
+	bucket = disk_lat_hist_bucket(nsec);
+	disk_lat_hist_record(part, sgrp, bucket);
+
+	whole = bdev_whole(part);
+	if (whole != part)
+		disk_lat_hist_record(whole, sgrp, bucket);
+}
+
+static void disk_lat_hist_seq_show(struct seq_file *seqf,
+				   struct block_device *bdev)
+{
+	u64 buckets[NR_STAT_GROUPS][DISK_LAT_HIST_BUCKETS] = { };
+	int cpu, sgrp, i, bucket;
+
+	if (!bdev->bd_lat_hist)
+		return;
+
+	for_each_possible_cpu(cpu) {
+		struct disk_lat_hist *hist = per_cpu_ptr(bdev->bd_lat_hist, cpu);
+
+		for (sgrp = 0; sgrp < NR_STAT_GROUPS; sgrp++)
+			for (i = 0; i < DISK_LAT_HIST_BUCKETS; i++)
+				buckets[sgrp][i] += hist->buckets[sgrp][i];
+	}
+
+	for (i = 0; i < NR_STAT_GROUPS; i++) {
+		sgrp = disk_lat_hist_order[i];
+		seq_printf(seqf, "%4d %7d %pg",
+			   MAJOR(bdev->bd_dev), MINOR(bdev->bd_dev), bdev);
+		for (bucket = 0; bucket < DISK_LAT_HIST_BUCKETS; bucket++)
+			seq_printf(seqf, " %llu", buckets[sgrp][bucket]);
+		seq_putc(seqf, '\n');
+	}
+}
+
+int disk_lat_buckets_show(struct seq_file *seqf, void *v)
+{
+	int i;
+
+	for (i = 0; i < DISK_LAT_HIST_BOUNDS; i++)
+		seq_printf(seqf, "%s%llu", i ? " " : "",
+			   disk_lat_hist_bounds_us[i]);
+	seq_putc(seqf, '\n');
+
+	return 0;
+}
+
+int disk_lat_hists_show(struct seq_file *seqf, void *v)
+{
+	struct gendisk *disk = v;
+	struct block_device *part;
+	unsigned long idx;
+
+	rcu_read_lock();
+	xa_for_each(&disk->part_tbl, idx, part) {
+		if (bdev_is_partition(part) && !bdev_nr_sectors(part))
+			continue;
+		disk_lat_hist_seq_show(seqf, part);
+	}
+	rcu_read_unlock();
+
+	return 0;
+}
diff --git a/block/genhd.c b/block/genhd.c
index f84b6a355b574..b4ee18f5c4ea2 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -747,6 +747,7 @@ static void __del_gendisk(struct gendisk *disk)
 	disk->slave_dir = NULL;
 
 	part_stat_set_all(disk->part0, 0);
+	disk_lat_hist_set_all(disk->part0, 0);
 	disk->part0->bd_stamp = 0;
 	sysfs_remove_link(block_depr, dev_name(disk_to_dev(disk)));
 	pm_runtime_set_memalloc_noio(disk_to_dev(disk), false);
@@ -1420,9 +1421,18 @@ static const struct seq_operations diskstats_op = {
 	.show	= diskstats_show
 };
 
+static const struct seq_operations disk_lat_hists_op = {
+	.start	= disk_seqf_start,
+	.next	= disk_seqf_next,
+	.stop	= disk_seqf_stop,
+	.show	= disk_lat_hists_show
+};
+
 static int __init proc_genhd_init(void)
 {
 	proc_create_seq("diskstats", 0, NULL, &diskstats_op);
+	proc_create_single("disk_lat_buckets", 0, NULL, disk_lat_buckets_show);
+	proc_create_seq("disk_lat_hists", 0, NULL, &disk_lat_hists_op);
 	proc_create_seq("partitions", 0, NULL, &partitions_op);
 	return 0;
 }
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 8808ee76e73c0..be2d31aea5d44 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -44,6 +44,7 @@ struct block_device {
 	struct gendisk *	bd_disk;
 	struct request_queue *	bd_queue;
 	struct disk_stats __percpu *bd_stats;
+	struct disk_lat_hist __percpu *bd_lat_hist;
 	unsigned long		bd_stamp;
 	atomic_t		__bd_flags;	// partition number + flags
 #define BD_PARTNO		255	// lower 8 bits; assign-once
-- 
2.39.5