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>

1 files changed, 248 insertions, 0 deletions

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;
+}