block: add scalable completion tracking of requests

For legacy block, we simply track them in the request queue. For
blk-mq, we track them on a per-sw queue basis, which we can then
sum up through the hardware queues and finally to a per device
state.

The stats are tracked in, roughly, 0.1s interval windows.

Add sysfs files to display the stats.

The feature is off by default, to avoid any extra overhead. In-kernel
users of it can turn it on by setting QUEUE_FLAG_STATS in the queue
flags. We currently don't turn it on if someone just reads any of
the stats files, that is something we could add as well.

Signed-off-by: Jens Axboe <axboe@fb.com>

8 files changed, 404 insertions, 3 deletions

diff --git a/block/Makefile b/block/Makefile
index 934dac73fb37..2528c596f7ec 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -5,7 +5,7 @@
 obj-$(CONFIG_BLOCK) := bio.o elevator.o blk-core.o blk-tag.o blk-sysfs.o \
 			blk-flush.o blk-settings.o blk-ioc.o blk-map.o \
 			blk-exec.o blk-merge.o blk-softirq.o blk-timeout.o \
-			blk-lib.o blk-mq.o blk-mq-tag.o \
+			blk-lib.o blk-mq.o blk-mq-tag.o blk-stat.o \
 			blk-mq-sysfs.o blk-mq-cpumap.o ioctl.o \
 			genhd.o scsi_ioctl.o partition-generic.o ioprio.o \
 			badblocks.o partitions/
diff --git a/block/blk-core.c b/block/blk-core.c
index 2deca48a4a05..216372b01624 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2464,6 +2464,11 @@ void blk_start_request(struct request *req)
 {
 	blk_dequeue_request(req);
 
+	if (test_bit(QUEUE_FLAG_STATS, &req->q->queue_flags)) {
+		blk_stat_set_issue_time(&req->issue_stat);
+		req->rq_flags |= RQF_STATS;
+	}
+
 	/*
 	 * We are now handing the request to the hardware, initialize
 	 * resid_len to full count and add the timeout handler.
@@ -2683,8 +2688,13 @@ EXPORT_SYMBOL_GPL(blk_unprep_request);
  */
 void blk_finish_request(struct request *req, int error)
 {
+	struct request_queue *q = req->q;
+
+	if (req->rq_flags & RQF_STATS)
+		blk_stat_add(&q->rq_stats[rq_data_dir(req)], req);
+
 	if (req->rq_flags & RQF_QUEUED)
-		blk_queue_end_tag(req->q, req);
+		blk_queue_end_tag(q, req);
 
 	BUG_ON(blk_queued_rq(req));
 
@@ -2704,7 +2714,7 @@ void blk_finish_request(struct request *req, int error)
 		if (blk_bidi_rq(req))
 			__blk_put_request(req->next_rq->q, req->next_rq);
 
-		__blk_put_request(req->q, req);
+		__blk_put_request(q, req);
 	}
 }
 EXPORT_SYMBOL(blk_finish_request);
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 01fb455d3377..eacd3af72099 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -259,6 +259,47 @@ static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page)
 	return ret;
 }
 
+static void blk_mq_stat_clear(struct blk_mq_hw_ctx *hctx)
+{
+	struct blk_mq_ctx *ctx;
+	unsigned int i;
+
+	hctx_for_each_ctx(hctx, ctx, i) {
+		blk_stat_init(&ctx->stat[BLK_STAT_READ]);
+		blk_stat_init(&ctx->stat[BLK_STAT_WRITE]);
+	}
+}
+
+static ssize_t blk_mq_hw_sysfs_stat_store(struct blk_mq_hw_ctx *hctx,
+					  const char *page, size_t count)
+{
+	blk_mq_stat_clear(hctx);
+	return count;
+}
+
+static ssize_t print_stat(char *page, struct blk_rq_stat *stat, const char *pre)
+{
+	return sprintf(page, "%s samples=%llu, mean=%lld, min=%lld, max=%lld\n",
+			pre, (long long) stat->nr_samples,
+			(long long) stat->mean, (long long) stat->min,
+			(long long) stat->max);
+}
+
+static ssize_t blk_mq_hw_sysfs_stat_show(struct blk_mq_hw_ctx *hctx, char *page)
+{
+	struct blk_rq_stat stat[2];
+	ssize_t ret;
+
+	blk_stat_init(&stat[BLK_STAT_READ]);
+	blk_stat_init(&stat[BLK_STAT_WRITE]);
+
+	blk_hctx_stat_get(hctx, stat);
+
+	ret = print_stat(page, &stat[BLK_STAT_READ], "read :");
+	ret += print_stat(page + ret, &stat[BLK_STAT_WRITE], "write:");
+	return ret;
+}
+
 static struct blk_mq_ctx_sysfs_entry blk_mq_sysfs_dispatched = {
 	.attr = {.name = "dispatched", .mode = S_IRUGO },
 	.show = blk_mq_sysfs_dispatched_show,
@@ -317,6 +358,11 @@ static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_poll = {
 	.show = blk_mq_hw_sysfs_poll_show,
 	.store = blk_mq_hw_sysfs_poll_store,
 };
+static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_stat = {
+	.attr = {.name = "stats", .mode = S_IRUGO | S_IWUSR },
+	.show = blk_mq_hw_sysfs_stat_show,
+	.store = blk_mq_hw_sysfs_stat_store,
+};
 
 static struct attribute *default_hw_ctx_attrs[] = {
 	&blk_mq_hw_sysfs_queued.attr,
@@ -327,6 +373,7 @@ static struct attribute *default_hw_ctx_attrs[] = {
 	&blk_mq_hw_sysfs_cpus.attr,
 	&blk_mq_hw_sysfs_active.attr,
 	&blk_mq_hw_sysfs_poll.attr,
+	&blk_mq_hw_sysfs_stat.attr,
 	NULL,
 };
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 6f5cb3f3dcac..19795886d46e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -30,6 +30,7 @@
 #include "blk.h"
 #include "blk-mq.h"
 #include "blk-mq-tag.h"
+#include "blk-stat.h"
 
 static DEFINE_MUTEX(all_q_mutex);
 static LIST_HEAD(all_q_list);
@@ -403,10 +404,27 @@ static void blk_mq_ipi_complete_request(struct request *rq)
 	put_cpu();
 }
 
+static void blk_mq_stat_add(struct request *rq)
+{
+	if (rq->rq_flags & RQF_STATS) {
+		/*
+		 * We could rq->mq_ctx here, but there's less of a risk
+		 * of races if we have the completion event add the stats
+		 * to the local software queue.
+		 */
+		struct blk_mq_ctx *ctx;
+
+		ctx = __blk_mq_get_ctx(rq->q, raw_smp_processor_id());
+		blk_stat_add(&ctx->stat[rq_data_dir(rq)], rq);
+	}
+}
+
 static void __blk_mq_complete_request(struct request *rq)
 {
 	struct request_queue *q = rq->q;
 
+	blk_mq_stat_add(rq);
+
 	if (!q->softirq_done_fn)
 		blk_mq_end_request(rq, rq->errors);
 	else
@@ -450,6 +468,11 @@ void blk_mq_start_request(struct request *rq)
 	if (unlikely(blk_bidi_rq(rq)))
 		rq->next_rq->resid_len = blk_rq_bytes(rq->next_rq);
 
+	if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
+		blk_stat_set_issue_time(&rq->issue_stat);
+		rq->rq_flags |= RQF_STATS;
+	}
+
 	blk_add_timer(rq);
 
 	/*
@@ -1784,6 +1807,8 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
 		spin_lock_init(&__ctx->lock);
 		INIT_LIST_HEAD(&__ctx->rq_list);
 		__ctx->queue = q;
+		blk_stat_init(&__ctx->stat[BLK_STAT_READ]);
+		blk_stat_init(&__ctx->stat[BLK_STAT_WRITE]);
 
 		/* If the cpu isn't online, the cpu is mapped to first hctx */
 		if (!cpu_online(i))
diff --git a/block/blk-mq.h b/block/blk-mq.h
index ac772dac7ce8..b444370ae05b 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -1,6 +1,8 @@
 #ifndef INT_BLK_MQ_H
 #define INT_BLK_MQ_H
 
+#include "blk-stat.h"
+
 struct blk_mq_tag_set;
 
 struct blk_mq_ctx {
@@ -18,6 +20,7 @@ struct blk_mq_ctx {
 
 	/* incremented at completion time */
 	unsigned long		____cacheline_aligned_in_smp rq_completed[2];
+	struct blk_rq_stat	stat[2];
 
 	struct request_queue	*queue;
 	struct kobject		kobj;
diff --git a/block/blk-stat.c b/block/blk-stat.c
new file mode 100644
index 000000000000..688c958367ee
--- /dev/null
+++ b/block/blk-stat.c
@@ -0,0 +1,248 @@
+/*
+ * Block stat tracking code
+ *
+ * Copyright (C) 2016 Jens Axboe
+ */
+#include <linux/kernel.h>
+#include <linux/blk-mq.h>
+
+#include "blk-stat.h"
+#include "blk-mq.h"
+
+static void blk_stat_flush_batch(struct blk_rq_stat *stat)
+{
+	const s32 nr_batch = READ_ONCE(stat->nr_batch);
+	const s32 nr_samples = READ_ONCE(stat->nr_batch);
+
+	if (!nr_batch)
+		return;
+	if (!nr_samples)
+		stat->mean = div64_s64(stat->batch, nr_batch);
+	else {
+		stat->mean = div64_s64((stat->mean * nr_samples) +
+					stat->batch,
+					nr_batch + nr_samples);
+	}
+
+	stat->nr_samples += nr_batch;
+	stat->nr_batch = stat->batch = 0;
+}
+
+static void blk_stat_sum(struct blk_rq_stat *dst, struct blk_rq_stat *src)
+{
+	if (!src->nr_samples)
+		return;
+
+	blk_stat_flush_batch(src);
+
+	dst->min = min(dst->min, src->min);
+	dst->max = max(dst->max, src->max);
+
+	if (!dst->nr_samples)
+		dst->mean = src->mean;
+	else {
+		dst->mean = div64_s64((src->mean * src->nr_samples) +
+					(dst->mean * dst->nr_samples),
+					dst->nr_samples + src->nr_samples);
+	}
+	dst->nr_samples += src->nr_samples;
+}
+
+static void blk_mq_stat_get(struct request_queue *q, struct blk_rq_stat *dst)
+{
+	struct blk_mq_hw_ctx *hctx;
+	struct blk_mq_ctx *ctx;
+	uint64_t latest = 0;
+	int i, j, nr;
+
+	blk_stat_init(&dst[BLK_STAT_READ]);
+	blk_stat_init(&dst[BLK_STAT_WRITE]);
+
+	nr = 0;
+	do {
+		uint64_t newest = 0;
+
+		queue_for_each_hw_ctx(q, hctx, i) {
+			hctx_for_each_ctx(hctx, ctx, j) {
+				if (!ctx->stat[BLK_STAT_READ].nr_samples &&
+				    !ctx->stat[BLK_STAT_WRITE].nr_samples)
+					continue;
+				if (ctx->stat[BLK_STAT_READ].time > newest)
+					newest = ctx->stat[BLK_STAT_READ].time;
+				if (ctx->stat[BLK_STAT_WRITE].time > newest)
+					newest = ctx->stat[BLK_STAT_WRITE].time;
+			}
+		}
+
+		/*
+		 * No samples
+		 */
+		if (!newest)
+			break;
+
+		if (newest > latest)
+			latest = newest;
+
+		queue_for_each_hw_ctx(q, hctx, i) {
+			hctx_for_each_ctx(hctx, ctx, j) {
+				if (ctx->stat[BLK_STAT_READ].time == newest) {
+					blk_stat_sum(&dst[BLK_STAT_READ],
+						     &ctx->stat[BLK_STAT_READ]);
+					nr++;
+				}
+				if (ctx->stat[BLK_STAT_WRITE].time == newest) {
+					blk_stat_sum(&dst[BLK_STAT_WRITE],
+						     &ctx->stat[BLK_STAT_WRITE]);
+					nr++;
+				}
+			}
+		}
+		/*
+		 * If we race on finding an entry, just loop back again.
+		 * Should be very rare.
+		 */
+	} while (!nr);
+
+	dst[BLK_STAT_READ].time = dst[BLK_STAT_WRITE].time = latest;
+}
+
+void blk_queue_stat_get(struct request_queue *q, struct blk_rq_stat *dst)
+{
+	if (q->mq_ops)
+		blk_mq_stat_get(q, dst);
+	else {
+		memcpy(&dst[BLK_STAT_READ], &q->rq_stats[BLK_STAT_READ],
+				sizeof(struct blk_rq_stat));
+		memcpy(&dst[BLK_STAT_WRITE], &q->rq_stats[BLK_STAT_WRITE],
+				sizeof(struct blk_rq_stat));
+	}
+}
+
+void blk_hctx_stat_get(struct blk_mq_hw_ctx *hctx, struct blk_rq_stat *dst)
+{
+	struct blk_mq_ctx *ctx;
+	unsigned int i, nr;
+
+	nr = 0;
+	do {
+		uint64_t newest = 0;
+
+		hctx_for_each_ctx(hctx, ctx, i) {
+			if (!ctx->stat[BLK_STAT_READ].nr_samples &&
+			    !ctx->stat[BLK_STAT_WRITE].nr_samples)
+				continue;
+
+			if (ctx->stat[BLK_STAT_READ].time > newest)
+				newest = ctx->stat[BLK_STAT_READ].time;
+			if (ctx->stat[BLK_STAT_WRITE].time > newest)
+				newest = ctx->stat[BLK_STAT_WRITE].time;
+		}
+
+		if (!newest)
+			break;
+
+		hctx_for_each_ctx(hctx, ctx, i) {
+			if (ctx->stat[BLK_STAT_READ].time == newest) {
+				blk_stat_sum(&dst[BLK_STAT_READ],
+						&ctx->stat[BLK_STAT_READ]);
+				nr++;
+			}
+			if (ctx->stat[BLK_STAT_WRITE].time == newest) {
+				blk_stat_sum(&dst[BLK_STAT_WRITE],
+						&ctx->stat[BLK_STAT_WRITE]);
+				nr++;
+			}
+		}
+		/*
+		 * If we race on finding an entry, just loop back again.
+		 * Should be very rare, as the window is only updated
+		 * occasionally
+		 */
+	} while (!nr);
+}
+
+static void __blk_stat_init(struct blk_rq_stat *stat, s64 time_now)
+{
+	stat->min = -1ULL;
+	stat->max = stat->nr_samples = stat->mean = 0;
+	stat->batch = stat->nr_batch = 0;
+	stat->time = time_now & BLK_STAT_NSEC_MASK;
+}
+
+void blk_stat_init(struct blk_rq_stat *stat)
+{
+	__blk_stat_init(stat, ktime_to_ns(ktime_get()));
+}
+
+static bool __blk_stat_is_current(struct blk_rq_stat *stat, s64 now)
+{
+	return (now & BLK_STAT_NSEC_MASK) == (stat->time & BLK_STAT_NSEC_MASK);
+}
+
+bool blk_stat_is_current(struct blk_rq_stat *stat)
+{
+	return __blk_stat_is_current(stat, ktime_to_ns(ktime_get()));
+}
+
+void blk_stat_add(struct blk_rq_stat *stat, struct request *rq)
+{
+	s64 now, value;
+
+	now = __blk_stat_time(ktime_to_ns(ktime_get()));
+	if (now < blk_stat_time(&rq->issue_stat))
+		return;
+
+	if (!__blk_stat_is_current(stat, now))
+		__blk_stat_init(stat, now);
+
+	value = now - blk_stat_time(&rq->issue_stat);
+	if (value > stat->max)
+		stat->max = value;
+	if (value < stat->min)
+		stat->min = value;
+
+	if (stat->batch + value < stat->batch ||
+	    stat->nr_batch + 1 == BLK_RQ_STAT_BATCH)
+		blk_stat_flush_batch(stat);
+
+	stat->batch += value;
+	stat->nr_batch++;
+}
+
+void blk_stat_clear(struct request_queue *q)
+{
+	if (q->mq_ops) {
+		struct blk_mq_hw_ctx *hctx;
+		struct blk_mq_ctx *ctx;
+		int i, j;
+
+		queue_for_each_hw_ctx(q, hctx, i) {
+			hctx_for_each_ctx(hctx, ctx, j) {
+				blk_stat_init(&ctx->stat[BLK_STAT_READ]);
+				blk_stat_init(&ctx->stat[BLK_STAT_WRITE]);
+			}
+		}
+	} else {
+		blk_stat_init(&q->rq_stats[BLK_STAT_READ]);
+		blk_stat_init(&q->rq_stats[BLK_STAT_WRITE]);
+	}
+}
+
+void blk_stat_set_issue_time(struct blk_issue_stat *stat)
+{
+	stat->time = (stat->time & BLK_STAT_MASK) |
+			(ktime_to_ns(ktime_get()) & BLK_STAT_TIME_MASK);
+}
+
+/*
+ * Enable stat tracking, return whether it was enabled
+ */
+bool blk_stat_enable(struct request_queue *q)
+{
+	if (!test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
+		set_bit(QUEUE_FLAG_STATS, &q->queue_flags);
+		return false;
+	}
+
+	return true;
+}
diff --git a/block/blk-stat.h b/block/blk-stat.h
new file mode 100644
index 000000000000..a2050a0a5314
--- /dev/null
+++ b/block/blk-stat.h
@@ -0,0 +1,42 @@
+#ifndef BLK_STAT_H
+#define BLK_STAT_H
+
+/*
+ * ~0.13s window as a power-of-2 (2^27 nsecs)
+ */
+#define BLK_STAT_NSEC		134217728ULL
+#define BLK_STAT_NSEC_MASK	~(BLK_STAT_NSEC - 1)
+
+/*
+ * Upper 3 bits can be used elsewhere
+ */
+#define BLK_STAT_RES_BITS	3
+#define BLK_STAT_SHIFT		(64 - BLK_STAT_RES_BITS)
+#define BLK_STAT_TIME_MASK	((1ULL << BLK_STAT_SHIFT) - 1)
+#define BLK_STAT_MASK		~BLK_STAT_TIME_MASK
+
+enum {
+	BLK_STAT_READ	= 0,
+	BLK_STAT_WRITE,
+};
+
+void blk_stat_add(struct blk_rq_stat *, struct request *);
+void blk_hctx_stat_get(struct blk_mq_hw_ctx *, struct blk_rq_stat *);
+void blk_queue_stat_get(struct request_queue *, struct blk_rq_stat *);
+void blk_stat_clear(struct request_queue *);
+void blk_stat_init(struct blk_rq_stat *);
+bool blk_stat_is_current(struct blk_rq_stat *);
+void blk_stat_set_issue_time(struct blk_issue_stat *);
+bool blk_stat_enable(struct request_queue *);
+
+static inline u64 __blk_stat_time(u64 time)
+{
+	return time & BLK_STAT_TIME_MASK;
+}
+
+static inline u64 blk_stat_time(struct blk_issue_stat *stat)
+{
+	return __blk_stat_time(stat->time);
+}
+
+#endif
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 488c2e28feb8..9cdb7247727a 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -401,6 +401,26 @@ static ssize_t queue_dax_show(struct request_queue *q, char *page)
 	return queue_var_show(blk_queue_dax(q), page);
 }
 
+static ssize_t print_stat(char *page, struct blk_rq_stat *stat, const char *pre)
+{
+	return sprintf(page, "%s samples=%llu, mean=%lld, min=%lld, max=%lld\n",
+			pre, (long long) stat->nr_samples,
+			(long long) stat->mean, (long long) stat->min,
+			(long long) stat->max);
+}
+
+static ssize_t queue_stats_show(struct request_queue *q, char *page)
+{
+	struct blk_rq_stat stat[2];
+	ssize_t ret;
+
+	blk_queue_stat_get(q, stat);
+
+	ret = print_stat(page, &stat[BLK_STAT_READ], "read :");
+	ret += print_stat(page + ret, &stat[BLK_STAT_WRITE], "write:");
+	return ret;
+}
+
 static struct queue_sysfs_entry queue_requests_entry = {
 	.attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
 	.show = queue_requests_show,
@@ -553,6 +573,11 @@ static struct queue_sysfs_entry queue_dax_entry = {
 	.show = queue_dax_show,
 };
 
+static struct queue_sysfs_entry queue_stats_entry = {
+	.attr = {.name = "stats", .mode = S_IRUGO },
+	.show = queue_stats_show,
+};
+
 static struct attribute *default_attrs[] = {
 	&queue_requests_entry.attr,
 	&queue_ra_entry.attr,
@@ -582,6 +607,7 @@ static struct attribute *default_attrs[] = {
 	&queue_poll_entry.attr,
 	&queue_wc_entry.attr,
 	&queue_dax_entry.attr,
+	&queue_stats_entry.attr,
 	NULL,
 };