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Date: Thu, 16 Apr 2020 17:13:36 -0400
From: Gabriel Krisman Bertazi <krisman@...labora.com>
To: agk@...hat.com, snitzer@...hat.com
Cc: dm-devel@...hat.com, linux-kernel@...r.kernel.org, song@...nel.org,
khazhy@...gle.com, kernel@...labora.com,
Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: [PATCH 2/2] md: Add Historical Service Time Path Selector
From: Khazhismel Kumykov <khazhy@...gle.com>
This new selector keeps an exponential moving average of the service
time for each path (losely defined as delta between start_io and
end_io), and uses this along with the number of inflight requests to
estimate future service time for a path. Since we don't have a prober
to account for temporally slow paths, re-try "slow" paths every once in
a while (num_paths * historical_service_time). To account for fast paths
transitioning to slow, if a path has not completed any request within
(num_paths * historical_service_time), limit the number of outstanding
requests. To account for low volume situations where number of
inflights would be zero, the last finish time of each path is factored
in.
Signed-off-by: Khazhismel Kumykov <khazhy@...gle.com>
Co-developed-by: Gabriel Krisman Bertazi <krisman@...labora.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>
---
drivers/md/Kconfig | 11 +
drivers/md/Makefile | 1 +
drivers/md/dm-historical-service-time.c | 561 ++++++++++++++++++++++++
3 files changed, 573 insertions(+)
create mode 100644 drivers/md/dm-historical-service-time.c
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index d6d5ab23c088..6f348a66450c 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -443,6 +443,17 @@ config DM_MULTIPATH_ST
If unsure, say N.
+config DM_MULTIPATH_HST
+ tristate "I/O Path Selector based on historical service time"
+ depends on DM_MULTIPATH
+ help
+ This path selector is a dynamic load balancer which selects
+ the path expected to complete the incoming I/O in the shortest
+ time by comparing estimated service time (based on historical
+ service time).
+
+ If unsure, say N.
+
config DM_DELAY
tristate "I/O delaying target"
depends on BLK_DEV_DM
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index d91a7edcd2ab..d148eeade973 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -54,6 +54,7 @@ obj-$(CONFIG_DM_FLAKEY) += dm-flakey.o
obj-$(CONFIG_DM_MULTIPATH) += dm-multipath.o dm-round-robin.o
obj-$(CONFIG_DM_MULTIPATH_QL) += dm-queue-length.o
obj-$(CONFIG_DM_MULTIPATH_ST) += dm-service-time.o
+obj-$(CONFIG_DM_MULTIPATH_HST) += dm-historical-service-time.o
obj-$(CONFIG_DM_SWITCH) += dm-switch.o
obj-$(CONFIG_DM_SNAPSHOT) += dm-snapshot.o
obj-$(CONFIG_DM_PERSISTENT_DATA) += persistent-data/
diff --git a/drivers/md/dm-historical-service-time.c b/drivers/md/dm-historical-service-time.c
new file mode 100644
index 000000000000..96bab52bba94
--- /dev/null
+++ b/drivers/md/dm-historical-service-time.c
@@ -0,0 +1,561 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Historical Service Time
+ *
+ * Keeps a time-weighted exponential moving average of the historical
+ * service time. Estimates future service time based on the historical
+ * service time and the number of outstanding requests.
+ *
+ * Marks paths stale if they have not finished within hst *
+ * num_paths. If a path is stale and unused, we will send a single
+ * request to probe in case the path has improved. This situation
+ * generally arises if the path is so much worse than others that it
+ * will never have the best estimated service time, or if the entire
+ * multipath device is unused. If a path is stale and in use, limit the
+ * number of requests it can receive with the assumption that the path
+ * has become degraded.
+ *
+ * To avoid repeatedly calculating exponents for time weighting, times
+ * are split into HST_WEIGHT_COUNT buckets each (1 >> HST_BUCKET_SHIFT)
+ * ns, and the weighting is pre-calculated.
+ *
+ */
+
+#include "dm.h"
+#include "dm-path-selector.h"
+
+#include <linux/blkdev.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+
+
+#define DM_MSG_PREFIX "multipath historical-service-time"
+#define HST_MIN_IO 1
+#define HST_VERSION "0.1.1"
+
+#define HST_FIXED_SHIFT 10 /* 10 bits of decimal precision */
+#define HST_FIXED_MAX (ULLONG_MAX >> HST_FIXED_SHIFT)
+#define HST_FIXED_1 (1 << HST_FIXED_SHIFT)
+#define HST_FIXED_95 972
+#define HST_MAX_INFLIGHT HST_FIXED_1
+#define HST_BUCKET_SHIFT 24 /* Buckets are ~ 16ms */
+#define HST_WEIGHT_COUNT 64ULL
+
+struct selector {
+ struct list_head valid_paths;
+ struct list_head failed_paths;
+ int valid_count;
+ spinlock_t lock;
+
+ unsigned int weights[HST_WEIGHT_COUNT];
+ unsigned int threshold_multiplier;
+};
+
+struct path_info {
+ struct list_head list;
+ struct dm_path *path;
+ unsigned int repeat_count;
+
+ spinlock_t lock;
+
+ u64 historical_service_time; /* Fixed point */
+
+ u64 stale_after;
+ u64 last_finish;
+
+ u64 outstanding;
+};
+
+/**
+ * fixed_power - compute: x^n, in O(log n) time
+ *
+ * @x: base of the power
+ * @frac_bits: fractional bits of @x
+ * @n: power to raise @x to.
+ *
+ * By exploiting the relation between the definition of the natural power
+ * function: x^n := x*x*...*x (x multiplied by itself for n times), and
+ * the binary encoding of numbers used by computers: n := \Sum n_i * 2^i,
+ * (where: n_i \elem {0, 1}, the binary vector representing n),
+ * we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is
+ * of course trivially computable in O(log_2 n), the length of our binary
+ * vector.
+ *
+ * (see: kernel/sched/loadavg.c)
+ */
+static u64 fixed_power(u64 x, unsigned int frac_bits, unsigned int n)
+{
+ unsigned long result = 1UL << frac_bits;
+
+ if (n) {
+ for (;;) {
+ if (n & 1) {
+ result *= x;
+ result += 1UL << (frac_bits - 1);
+ result >>= frac_bits;
+ }
+ n >>= 1;
+ if (!n)
+ break;
+ x *= x;
+ x += 1UL << (frac_bits - 1);
+ x >>= frac_bits;
+ }
+ }
+
+ return result;
+}
+
+/*
+ * Calculate the next value of an exponential moving average
+ * a_1 = a_0 * e + a * (1 - e)
+ *
+ * @last: [0, ULLONG_MAX >> HST_FIXED_SHIFT]
+ * @next: [0, ULLONG_MAX >> HST_FIXED_SHIFT]
+ * @weight: [0, HST_FIXED_1]
+ *
+ * Note:
+ * To account for multiple periods in the same calculation,
+ * a_n = a_0 * e^n + a * (1 - e^n),
+ * so call fixed_ema(last, next, pow(weight, N))
+ */
+static u64 fixed_ema(u64 last, u64 next, u64 weight)
+{
+ last *= weight;
+ last += next * (HST_FIXED_1 - weight);
+ last += 1ULL << (HST_FIXED_SHIFT - 1);
+ return last >> HST_FIXED_SHIFT;
+}
+
+static struct selector *alloc_selector(void)
+{
+ struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
+
+ if (s) {
+ INIT_LIST_HEAD(&s->valid_paths);
+ INIT_LIST_HEAD(&s->failed_paths);
+ spin_lock_init(&s->lock);
+ s->valid_count = 0;
+ }
+
+ return s;
+}
+
+/*
+ * Get the weight for a given time span.
+ */
+static u64 hst_weight(struct path_selector *ps, u64 delta)
+{
+ struct selector *s = ps->context;
+ int bucket = clamp(delta >> HST_BUCKET_SHIFT, 0ULL,
+ HST_WEIGHT_COUNT - 1);
+
+ return s->weights[bucket];
+}
+
+/*
+ * Set up the weights array.
+ *
+ * weights[len-1] = 0
+ * weights[n] = base ^ (n + 1)
+ */
+static void hst_set_weights(struct path_selector *ps, unsigned int base)
+{
+ struct selector *s = ps->context;
+ int i;
+
+ if (base >= HST_FIXED_1)
+ return;
+
+ for (i = 0; i < HST_WEIGHT_COUNT - 1; i++)
+ s->weights[i] = fixed_power(base, HST_FIXED_SHIFT, i + 1);
+ s->weights[HST_WEIGHT_COUNT - 1] = 0;
+}
+
+static int hst_create(struct path_selector *ps, unsigned int argc, char **argv)
+{
+ struct selector *s;
+ unsigned int base_weight = HST_FIXED_95;
+ unsigned int threshold_multiplier = 0;
+ char dummy;
+
+ /*
+ * Arguments: [<base_weight> [<threshold_multiplier>]]
+ * <base_weight>: Base weight for ema [0, 1024) 10-bit fixed point. A
+ * value of 0 will completely ignore any history.
+ * If not given, default (HST_FIXED_95) is used.
+ * <threshold_multiplier>: Minimum threshold multiplier for paths to
+ * be considered different. That is, a path is
+ * considered different iff (p1 > N * p2) where p1
+ * is the path with higher service time. A threshold
+ * of 1 or 0 has no effect. Defaults to 0.
+ */
+ if (argc > 2)
+ return -EINVAL;
+
+ if (argc && (sscanf(argv[0], "%u%c", &base_weight, &dummy) != 1 ||
+ base_weight >= HST_FIXED_1)) {
+ return -EINVAL;
+ }
+
+ if (argc > 1 && (sscanf(argv[1], "%u%c",
+ &threshold_multiplier, &dummy) != 1)) {
+ return -EINVAL;
+ }
+
+ s = alloc_selector();
+ if (!s)
+ return -ENOMEM;
+
+ ps->context = s;
+
+ hst_set_weights(ps, base_weight);
+ s->threshold_multiplier = threshold_multiplier;
+ return 0;
+}
+
+static void free_paths(struct list_head *paths)
+{
+ struct path_info *pi, *next;
+
+ list_for_each_entry_safe(pi, next, paths, list) {
+ list_del(&pi->list);
+ kfree(pi);
+ }
+}
+
+static void hst_destroy(struct path_selector *ps)
+{
+ struct selector *s = ps->context;
+
+ free_paths(&s->valid_paths);
+ free_paths(&s->failed_paths);
+ kfree(s);
+ ps->context = NULL;
+}
+
+static int hst_status(struct path_selector *ps, struct dm_path *path,
+ status_type_t type, char *result, unsigned int maxlen)
+{
+ unsigned int sz = 0;
+ struct path_info *pi;
+
+ if (!path) {
+ struct selector *s = ps->context;
+
+ DMEMIT("2 %u %u ", s->weights[0], s->threshold_multiplier);
+ } else {
+ pi = path->pscontext;
+
+ switch (type) {
+ case STATUSTYPE_INFO:
+ DMEMIT("%llu %llu %llu ", pi->historical_service_time,
+ pi->outstanding, pi->stale_after);
+ break;
+ case STATUSTYPE_TABLE:
+ DMEMIT("0 ");
+ break;
+ }
+ }
+
+ return sz;
+}
+
+static int hst_add_path(struct path_selector *ps, struct dm_path *path,
+ int argc, char **argv, char **error)
+{
+ struct selector *s = ps->context;
+ struct path_info *pi;
+ unsigned int repeat_count = HST_MIN_IO;
+ char dummy;
+ unsigned long flags;
+
+ /*
+ * Arguments: [<repeat_count>]
+ * <repeat_count>: The number of I/Os before switching path.
+ * If not given, default (HST_MIN_IO) is used.
+ */
+ if (argc > 1) {
+ *error = "historical-service-time ps: incorrect number of arguments";
+ return -EINVAL;
+ }
+
+ if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
+ *error = "historical-service-time ps: invalid repeat count";
+ return -EINVAL;
+ }
+
+ /* allocate the path */
+ pi = kmalloc(sizeof(*pi), GFP_KERNEL);
+ if (!pi) {
+ *error = "historical-service-time ps: Error allocating path context";
+ return -ENOMEM;
+ }
+
+ pi->path = path;
+ pi->repeat_count = repeat_count;
+
+ pi->historical_service_time = HST_FIXED_1;
+
+ spin_lock_init(&pi->lock);
+ pi->outstanding = 0;
+
+ pi->stale_after = 0;
+ pi->last_finish = 0;
+
+ path->pscontext = pi;
+
+ spin_lock_irqsave(&s->lock, flags);
+ list_add_tail(&pi->list, &s->valid_paths);
+ s->valid_count++;
+ spin_unlock_irqrestore(&s->lock, flags);
+
+ return 0;
+}
+
+static void hst_fail_path(struct path_selector *ps, struct dm_path *path)
+{
+ struct selector *s = ps->context;
+ struct path_info *pi = path->pscontext;
+ unsigned long flags;
+
+ spin_lock_irqsave(&s->lock, flags);
+ list_move(&pi->list, &s->failed_paths);
+ s->valid_count--;
+ spin_unlock_irqrestore(&s->lock, flags);
+}
+
+static int hst_reinstate_path(struct path_selector *ps, struct dm_path *path)
+{
+ struct selector *s = ps->context;
+ struct path_info *pi = path->pscontext;
+ unsigned long flags;
+
+ spin_lock_irqsave(&s->lock, flags);
+ list_move_tail(&pi->list, &s->valid_paths);
+ s->valid_count++;
+ spin_unlock_irqrestore(&s->lock, flags);
+
+ return 0;
+}
+
+static void hst_fill_compare(struct path_info *pi, u64 *hst,
+ u64 *out, u64 *stale)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&pi->lock, flags);
+ *hst = pi->historical_service_time;
+ *out = pi->outstanding;
+ *stale = pi->stale_after;
+ spin_unlock_irqrestore(&pi->lock, flags);
+}
+
+/*
+ * Compare the estimated service time of 2 paths, pi1 and pi2,
+ * for the incoming I/O.
+ *
+ * Returns:
+ * < 0 : pi1 is better
+ * 0 : no difference between pi1 and pi2
+ * > 0 : pi2 is better
+ *
+ */
+static long long hst_compare(struct path_info *pi1, struct path_info *pi2,
+ u64 time_now, struct path_selector *ps)
+{
+ struct selector *s = ps->context;
+ u64 hst1, hst2;
+ long long out1, out2, stale1, stale2;
+ int pi2_better, over_threshold;
+
+ hst_fill_compare(pi1, &hst1, &out1, &stale1);
+ hst_fill_compare(pi2, &hst2, &out2, &stale2);
+
+ /* Check here if estimated latency for two paths are too similar.
+ * If this is the case, we skip extra calculation and just compare
+ * outstanding requests. In this case, any unloaded paths will
+ * be preferred.
+ */
+ if (hst1 > hst2)
+ over_threshold = hst1 > (s->threshold_multiplier * hst2);
+ else
+ over_threshold = hst2 > (s->threshold_multiplier * hst1);
+
+ if (!over_threshold)
+ return out1 - out2;
+
+ /*
+ * If an unloaded path is stale, choose it. If both paths are unloaded,
+ * choose path that is the most stale.
+ * (If one path is loaded, choose the other)
+ */
+ if ((!out1 && stale1 < time_now) || (!out2 && stale2 < time_now) ||
+ (!out1 && !out2))
+ return (!out2 * stale1) - (!out1 * stale2);
+
+ /* Compare estimated service time. If outstanding is the same, we
+ * don't need to multiply
+ */
+ if (out1 == out2) {
+ pi2_better = hst1 > hst2;
+ } else {
+ /* Potential overflow with out >= 1024 */
+ if (unlikely(out1 >= HST_MAX_INFLIGHT ||
+ out2 >= HST_MAX_INFLIGHT)) {
+ /* If over 1023 in-flights, we may overflow if hst
+ * is at max. (With this shift we still overflow at
+ * 1048576 in-flights, which is high enough).
+ */
+ hst1 >>= HST_FIXED_SHIFT;
+ hst2 >>= HST_FIXED_SHIFT;
+ }
+ pi2_better = (1 + out1) * hst1 > (1 + out2) * hst2;
+ }
+
+ /* In the case that the 'winner' is stale, limit to equal usage. */
+ if (pi2_better) {
+ if (stale2 < time_now)
+ return out1 - out2;
+ return 1;
+ }
+ if (stale1 < time_now)
+ return out1 - out2;
+ return -1;
+}
+
+static struct dm_path *hst_select_path(struct path_selector *ps,
+ size_t nr_bytes)
+{
+ struct selector *s = ps->context;
+ struct path_info *pi = NULL, *best = NULL;
+ u64 time_now = sched_clock();
+ struct dm_path *ret = NULL;
+ unsigned long flags;
+
+ spin_lock_irqsave(&s->lock, flags);
+ if (list_empty(&s->valid_paths))
+ goto out;
+
+ list_for_each_entry(pi, &s->valid_paths, list) {
+ if (!best || (hst_compare(pi, best, time_now, ps) < 0))
+ best = pi;
+ }
+
+ if (!best)
+ goto out;
+
+ /* Move last used path to end (least preferred in case of ties) */
+ list_move_tail(&best->list, &s->valid_paths);
+
+ ret = best->path;
+
+out:
+ spin_unlock_irqrestore(&s->lock, flags);
+ return ret;
+}
+
+static int hst_start_io(struct path_selector *ps, struct dm_path *path,
+ const struct request *rq, size_t nr_bytes)
+{
+ struct path_info *pi = path->pscontext;
+ unsigned long flags;
+
+ spin_lock_irqsave(&pi->lock, flags);
+ pi->outstanding++;
+ spin_unlock_irqrestore(&pi->lock, flags);
+
+ return 0;
+}
+
+static u64 path_service_time(struct path_info *pi, const struct request *rq)
+{
+ u64 start_time = rq->io_start_time_ns;
+ u64 sched_now = ktime_get_ns();
+
+ /* if a previous disk request has finished after this IO was
+ * sent to the hardware, pretend the submission happened
+ * serially.
+ */
+ if (time_after64(pi->last_finish, rq->io_start_time_ns))
+ start_time = pi->last_finish;
+
+ pi->last_finish = sched_now;
+ if (time_before64(sched_now, start_time))
+ return 0;
+
+ return sched_now - start_time;
+}
+
+static int hst_end_io(struct path_selector *ps, struct dm_path *path,
+ const struct request *rq, size_t nr_bytes)
+{
+ struct path_info *pi = path->pscontext;
+ struct selector *s = ps->context;
+ unsigned long flags;
+ u64 st;
+
+ spin_lock_irqsave(&pi->lock, flags);
+
+ st = path_service_time(pi, rq);
+ pi->outstanding--;
+ pi->historical_service_time =
+ fixed_ema(pi->historical_service_time,
+ min(st * HST_FIXED_1, HST_FIXED_MAX),
+ hst_weight(ps, st));
+
+ /*
+ * On request end, mark path as fresh. If a path hasn't
+ * finished any requests within the fresh period, the estimated
+ * service time is considered too optimistic and we limit the
+ * maximum requests on that path.
+ */
+ pi->stale_after = pi->last_finish +
+ (s->valid_count * (pi->historical_service_time >> HST_FIXED_SHIFT));
+ spin_unlock_irqrestore(&pi->lock, flags);
+
+ return 0;
+}
+
+static struct path_selector_type hst_ps = {
+ .name = "historical-service-time",
+ .module = THIS_MODULE,
+ .table_args = 1,
+ .info_args = 3,
+ .create = hst_create,
+ .destroy = hst_destroy,
+ .status = hst_status,
+ .add_path = hst_add_path,
+ .fail_path = hst_fail_path,
+ .reinstate_path = hst_reinstate_path,
+ .select_path = hst_select_path,
+ .start_io_rq = hst_start_io,
+ .end_io_rq = hst_end_io,
+};
+
+static int __init dm_hst_init(void)
+{
+ int r = dm_register_path_selector(&hst_ps);
+
+ if (r < 0)
+ DMERR("register failed %d", r);
+
+ DMINFO("version " HST_VERSION " loaded");
+
+ return r;
+}
+
+static void __exit dm_hst_exit(void)
+{
+ int r = dm_unregister_path_selector(&hst_ps);
+
+ if (r < 0)
+ DMERR("unregister failed %d", r);
+}
+
+module_init(dm_hst_init);
+module_exit(dm_hst_exit);
+
+MODULE_DESCRIPTION(DM_NAME " measured service time oriented path selector");
+MODULE_AUTHOR("Khazhismel Kumykov <khazhy@...gle.com>");
+MODULE_LICENSE("GPL");
--
2.26.0
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