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![Heinz Mauelshagen](/assets/img/avatar_default.png)
'GPL-2.0-only' is used instead of 'GPL-2.0' because SPDX has deprecated its use. Suggested-by: John Wiele <jwiele@redhat.com> Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
565 lines
14 KiB
C
565 lines
14 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* Historical Service Time
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*
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* Keeps a time-weighted exponential moving average of the historical
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* service time. Estimates future service time based on the historical
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* service time and the number of outstanding requests.
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*
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* Marks paths stale if they have not finished within hst *
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* num_paths. If a path is stale and unused, we will send a single
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* request to probe in case the path has improved. This situation
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* generally arises if the path is so much worse than others that it
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* will never have the best estimated service time, or if the entire
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* multipath device is unused. If a path is stale and in use, limit the
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* number of requests it can receive with the assumption that the path
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* has become degraded.
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*
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* To avoid repeatedly calculating exponents for time weighting, times
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* are split into HST_WEIGHT_COUNT buckets each (1 >> HST_BUCKET_SHIFT)
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* ns, and the weighting is pre-calculated.
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*
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*/
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#include "dm.h"
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#include "dm-path-selector.h"
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#include <linux/blkdev.h>
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#include <linux/slab.h>
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#include <linux/module.h>
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#define DM_MSG_PREFIX "multipath historical-service-time"
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#define HST_MIN_IO 1
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#define HST_VERSION "0.1.1"
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#define HST_FIXED_SHIFT 10 /* 10 bits of decimal precision */
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#define HST_FIXED_MAX (ULLONG_MAX >> HST_FIXED_SHIFT)
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#define HST_FIXED_1 (1 << HST_FIXED_SHIFT)
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#define HST_FIXED_95 972
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#define HST_MAX_INFLIGHT HST_FIXED_1
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#define HST_BUCKET_SHIFT 24 /* Buckets are ~ 16ms */
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#define HST_WEIGHT_COUNT 64ULL
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struct selector {
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struct list_head valid_paths;
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struct list_head failed_paths;
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int valid_count;
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spinlock_t lock;
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unsigned int weights[HST_WEIGHT_COUNT];
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unsigned int threshold_multiplier;
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};
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struct path_info {
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struct list_head list;
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struct dm_path *path;
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unsigned int repeat_count;
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spinlock_t lock;
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u64 historical_service_time; /* Fixed point */
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u64 stale_after;
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u64 last_finish;
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u64 outstanding;
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};
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/**
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* fixed_power - compute: x^n, in O(log n) time
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*
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* @x: base of the power
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* @frac_bits: fractional bits of @x
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* @n: power to raise @x to.
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*
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* By exploiting the relation between the definition of the natural power
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* function: x^n := x*x*...*x (x multiplied by itself for n times), and
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* the binary encoding of numbers used by computers: n := \Sum n_i * 2^i,
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* (where: n_i \elem {0, 1}, the binary vector representing n),
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* we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is
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* of course trivially computable in O(log_2 n), the length of our binary
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* vector.
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*
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* (see: kernel/sched/loadavg.c)
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*/
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static u64 fixed_power(u64 x, unsigned int frac_bits, unsigned int n)
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{
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unsigned long result = 1UL << frac_bits;
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if (n) {
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for (;;) {
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if (n & 1) {
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result *= x;
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result += 1UL << (frac_bits - 1);
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result >>= frac_bits;
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}
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n >>= 1;
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if (!n)
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break;
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x *= x;
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x += 1UL << (frac_bits - 1);
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x >>= frac_bits;
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}
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}
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return result;
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}
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/*
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* Calculate the next value of an exponential moving average
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* a_1 = a_0 * e + a * (1 - e)
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*
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* @last: [0, ULLONG_MAX >> HST_FIXED_SHIFT]
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* @next: [0, ULLONG_MAX >> HST_FIXED_SHIFT]
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* @weight: [0, HST_FIXED_1]
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*
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* Note:
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* To account for multiple periods in the same calculation,
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* a_n = a_0 * e^n + a * (1 - e^n),
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* so call fixed_ema(last, next, pow(weight, N))
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*/
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static u64 fixed_ema(u64 last, u64 next, u64 weight)
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{
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last *= weight;
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last += next * (HST_FIXED_1 - weight);
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last += 1ULL << (HST_FIXED_SHIFT - 1);
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return last >> HST_FIXED_SHIFT;
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}
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static struct selector *alloc_selector(void)
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{
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struct selector *s = kmalloc(sizeof(*s), GFP_KERNEL);
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if (s) {
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INIT_LIST_HEAD(&s->valid_paths);
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INIT_LIST_HEAD(&s->failed_paths);
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spin_lock_init(&s->lock);
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s->valid_count = 0;
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}
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return s;
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}
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/*
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* Get the weight for a given time span.
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*/
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static u64 hst_weight(struct path_selector *ps, u64 delta)
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{
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struct selector *s = ps->context;
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int bucket = clamp(delta >> HST_BUCKET_SHIFT, 0ULL,
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HST_WEIGHT_COUNT - 1);
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return s->weights[bucket];
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}
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/*
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* Set up the weights array.
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*
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* weights[len-1] = 0
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* weights[n] = base ^ (n + 1)
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*/
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static void hst_set_weights(struct path_selector *ps, unsigned int base)
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{
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struct selector *s = ps->context;
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int i;
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if (base >= HST_FIXED_1)
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return;
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for (i = 0; i < HST_WEIGHT_COUNT - 1; i++)
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s->weights[i] = fixed_power(base, HST_FIXED_SHIFT, i + 1);
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s->weights[HST_WEIGHT_COUNT - 1] = 0;
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}
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static int hst_create(struct path_selector *ps, unsigned int argc, char **argv)
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{
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struct selector *s;
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unsigned int base_weight = HST_FIXED_95;
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unsigned int threshold_multiplier = 0;
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char dummy;
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/*
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* Arguments: [<base_weight> [<threshold_multiplier>]]
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* <base_weight>: Base weight for ema [0, 1024) 10-bit fixed point. A
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* value of 0 will completely ignore any history.
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* If not given, default (HST_FIXED_95) is used.
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* <threshold_multiplier>: Minimum threshold multiplier for paths to
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* be considered different. That is, a path is
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* considered different iff (p1 > N * p2) where p1
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* is the path with higher service time. A threshold
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* of 1 or 0 has no effect. Defaults to 0.
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*/
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if (argc > 2)
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return -EINVAL;
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if (argc && (sscanf(argv[0], "%u%c", &base_weight, &dummy) != 1 ||
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base_weight >= HST_FIXED_1)) {
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return -EINVAL;
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}
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if (argc > 1 && (sscanf(argv[1], "%u%c",
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&threshold_multiplier, &dummy) != 1)) {
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return -EINVAL;
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}
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s = alloc_selector();
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if (!s)
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return -ENOMEM;
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ps->context = s;
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hst_set_weights(ps, base_weight);
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s->threshold_multiplier = threshold_multiplier;
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return 0;
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}
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static void free_paths(struct list_head *paths)
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{
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struct path_info *pi, *next;
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list_for_each_entry_safe(pi, next, paths, list) {
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list_del(&pi->list);
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kfree(pi);
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}
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}
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static void hst_destroy(struct path_selector *ps)
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{
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struct selector *s = ps->context;
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free_paths(&s->valid_paths);
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free_paths(&s->failed_paths);
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kfree(s);
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ps->context = NULL;
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}
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static int hst_status(struct path_selector *ps, struct dm_path *path,
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status_type_t type, char *result, unsigned int maxlen)
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{
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unsigned int sz = 0;
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struct path_info *pi;
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if (!path) {
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struct selector *s = ps->context;
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DMEMIT("2 %u %u ", s->weights[0], s->threshold_multiplier);
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} else {
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pi = path->pscontext;
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switch (type) {
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case STATUSTYPE_INFO:
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DMEMIT("%llu %llu %llu ", pi->historical_service_time,
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pi->outstanding, pi->stale_after);
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break;
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case STATUSTYPE_TABLE:
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DMEMIT("0 ");
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break;
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case STATUSTYPE_IMA:
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*result = '\0';
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break;
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}
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}
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return sz;
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}
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static int hst_add_path(struct path_selector *ps, struct dm_path *path,
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int argc, char **argv, char **error)
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{
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struct selector *s = ps->context;
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struct path_info *pi;
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unsigned int repeat_count = HST_MIN_IO;
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char dummy;
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unsigned long flags;
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/*
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* Arguments: [<repeat_count>]
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* <repeat_count>: The number of I/Os before switching path.
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* If not given, default (HST_MIN_IO) is used.
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*/
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if (argc > 1) {
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*error = "historical-service-time ps: incorrect number of arguments";
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return -EINVAL;
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}
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if (argc && (sscanf(argv[0], "%u%c", &repeat_count, &dummy) != 1)) {
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*error = "historical-service-time ps: invalid repeat count";
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return -EINVAL;
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}
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/* allocate the path */
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pi = kmalloc(sizeof(*pi), GFP_KERNEL);
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if (!pi) {
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*error = "historical-service-time ps: Error allocating path context";
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return -ENOMEM;
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}
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pi->path = path;
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pi->repeat_count = repeat_count;
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pi->historical_service_time = HST_FIXED_1;
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spin_lock_init(&pi->lock);
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pi->outstanding = 0;
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pi->stale_after = 0;
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pi->last_finish = 0;
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path->pscontext = pi;
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spin_lock_irqsave(&s->lock, flags);
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list_add_tail(&pi->list, &s->valid_paths);
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s->valid_count++;
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spin_unlock_irqrestore(&s->lock, flags);
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return 0;
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}
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static void hst_fail_path(struct path_selector *ps, struct dm_path *path)
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{
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struct selector *s = ps->context;
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struct path_info *pi = path->pscontext;
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unsigned long flags;
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spin_lock_irqsave(&s->lock, flags);
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list_move(&pi->list, &s->failed_paths);
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s->valid_count--;
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spin_unlock_irqrestore(&s->lock, flags);
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}
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static int hst_reinstate_path(struct path_selector *ps, struct dm_path *path)
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{
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struct selector *s = ps->context;
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struct path_info *pi = path->pscontext;
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unsigned long flags;
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spin_lock_irqsave(&s->lock, flags);
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list_move_tail(&pi->list, &s->valid_paths);
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s->valid_count++;
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spin_unlock_irqrestore(&s->lock, flags);
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return 0;
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}
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static void hst_fill_compare(struct path_info *pi, u64 *hst,
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u64 *out, u64 *stale)
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{
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unsigned long flags;
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spin_lock_irqsave(&pi->lock, flags);
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*hst = pi->historical_service_time;
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*out = pi->outstanding;
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*stale = pi->stale_after;
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spin_unlock_irqrestore(&pi->lock, flags);
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}
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/*
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* Compare the estimated service time of 2 paths, pi1 and pi2,
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* for the incoming I/O.
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*
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* Returns:
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* < 0 : pi1 is better
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* 0 : no difference between pi1 and pi2
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* > 0 : pi2 is better
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*
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*/
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static long long hst_compare(struct path_info *pi1, struct path_info *pi2,
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u64 time_now, struct path_selector *ps)
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{
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struct selector *s = ps->context;
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u64 hst1, hst2;
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long long out1, out2, stale1, stale2;
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int pi2_better, over_threshold;
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hst_fill_compare(pi1, &hst1, &out1, &stale1);
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hst_fill_compare(pi2, &hst2, &out2, &stale2);
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/* Check here if estimated latency for two paths are too similar.
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* If this is the case, we skip extra calculation and just compare
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* outstanding requests. In this case, any unloaded paths will
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* be preferred.
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*/
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if (hst1 > hst2)
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over_threshold = hst1 > (s->threshold_multiplier * hst2);
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else
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over_threshold = hst2 > (s->threshold_multiplier * hst1);
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if (!over_threshold)
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return out1 - out2;
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/*
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* If an unloaded path is stale, choose it. If both paths are unloaded,
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* choose path that is the most stale.
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* (If one path is loaded, choose the other)
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*/
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if ((!out1 && stale1 < time_now) || (!out2 && stale2 < time_now) ||
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(!out1 && !out2))
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return (!out2 * stale1) - (!out1 * stale2);
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/* Compare estimated service time. If outstanding is the same, we
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* don't need to multiply
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*/
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if (out1 == out2) {
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pi2_better = hst1 > hst2;
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} else {
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/* Potential overflow with out >= 1024 */
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if (unlikely(out1 >= HST_MAX_INFLIGHT ||
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out2 >= HST_MAX_INFLIGHT)) {
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/* If over 1023 in-flights, we may overflow if hst
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* is at max. (With this shift we still overflow at
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* 1048576 in-flights, which is high enough).
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*/
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hst1 >>= HST_FIXED_SHIFT;
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hst2 >>= HST_FIXED_SHIFT;
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}
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pi2_better = (1 + out1) * hst1 > (1 + out2) * hst2;
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}
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/* In the case that the 'winner' is stale, limit to equal usage. */
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if (pi2_better) {
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if (stale2 < time_now)
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return out1 - out2;
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return 1;
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}
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if (stale1 < time_now)
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return out1 - out2;
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return -1;
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}
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static struct dm_path *hst_select_path(struct path_selector *ps,
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size_t nr_bytes)
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{
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struct selector *s = ps->context;
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struct path_info *pi = NULL, *best = NULL;
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u64 time_now = ktime_get_ns();
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struct dm_path *ret = NULL;
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unsigned long flags;
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spin_lock_irqsave(&s->lock, flags);
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if (list_empty(&s->valid_paths))
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goto out;
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list_for_each_entry(pi, &s->valid_paths, list) {
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if (!best || (hst_compare(pi, best, time_now, ps) < 0))
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best = pi;
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}
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if (!best)
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goto out;
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/* Move last used path to end (least preferred in case of ties) */
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list_move_tail(&best->list, &s->valid_paths);
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ret = best->path;
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out:
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spin_unlock_irqrestore(&s->lock, flags);
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return ret;
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}
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static int hst_start_io(struct path_selector *ps, struct dm_path *path,
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size_t nr_bytes)
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{
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struct path_info *pi = path->pscontext;
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unsigned long flags;
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spin_lock_irqsave(&pi->lock, flags);
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pi->outstanding++;
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spin_unlock_irqrestore(&pi->lock, flags);
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return 0;
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}
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static u64 path_service_time(struct path_info *pi, u64 start_time)
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{
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u64 now = ktime_get_ns();
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/* if a previous disk request has finished after this IO was
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* sent to the hardware, pretend the submission happened
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* serially.
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*/
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if (time_after64(pi->last_finish, start_time))
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start_time = pi->last_finish;
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pi->last_finish = now;
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if (time_before64(now, start_time))
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return 0;
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return now - start_time;
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}
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static int hst_end_io(struct path_selector *ps, struct dm_path *path,
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size_t nr_bytes, u64 start_time)
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{
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struct path_info *pi = path->pscontext;
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struct selector *s = ps->context;
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unsigned long flags;
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u64 st;
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spin_lock_irqsave(&pi->lock, flags);
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st = path_service_time(pi, start_time);
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pi->outstanding--;
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pi->historical_service_time =
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fixed_ema(pi->historical_service_time,
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min(st * HST_FIXED_1, HST_FIXED_MAX),
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hst_weight(ps, st));
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/*
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* On request end, mark path as fresh. If a path hasn't
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* finished any requests within the fresh period, the estimated
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* service time is considered too optimistic and we limit the
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* maximum requests on that path.
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*/
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pi->stale_after = pi->last_finish +
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(s->valid_count * (pi->historical_service_time >> HST_FIXED_SHIFT));
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spin_unlock_irqrestore(&pi->lock, flags);
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return 0;
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}
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static struct path_selector_type hst_ps = {
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.name = "historical-service-time",
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.module = THIS_MODULE,
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.features = DM_PS_USE_HR_TIMER,
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.table_args = 1,
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.info_args = 3,
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.create = hst_create,
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.destroy = hst_destroy,
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.status = hst_status,
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.add_path = hst_add_path,
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.fail_path = hst_fail_path,
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.reinstate_path = hst_reinstate_path,
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.select_path = hst_select_path,
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.start_io = hst_start_io,
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.end_io = hst_end_io,
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};
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static int __init dm_hst_init(void)
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{
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int r = dm_register_path_selector(&hst_ps);
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if (r < 0)
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DMERR("register failed %d", r);
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DMINFO("version " HST_VERSION " loaded");
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return r;
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}
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static void __exit dm_hst_exit(void)
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{
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int r = dm_unregister_path_selector(&hst_ps);
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if (r < 0)
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DMERR("unregister failed %d", r);
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}
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module_init(dm_hst_init);
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module_exit(dm_hst_exit);
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MODULE_DESCRIPTION(DM_NAME " measured service time oriented path selector");
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MODULE_AUTHOR("Khazhismel Kumykov <khazhy@google.com>");
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MODULE_LICENSE("GPL");
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