| 1 | /* SPDX-License-Identifier: GPL-2.0 */ |
| 2 | /* |
| 3 | * Dynamic queue limits (dql) - Definitions |
| 4 | * |
| 5 | * Copyright (c) 2011, Tom Herbert <therbert@google.com> |
| 6 | * |
| 7 | * This header file contains the definitions for dynamic queue limits (dql). |
| 8 | * dql would be used in conjunction with a producer/consumer type queue |
| 9 | * (possibly a HW queue). Such a queue would have these general properties: |
| 10 | * |
| 11 | * 1) Objects are queued up to some limit specified as number of objects. |
| 12 | * 2) Periodically a completion process executes which retires consumed |
| 13 | * objects. |
| 14 | * 3) Starvation occurs when limit has been reached, all queued data has |
| 15 | * actually been consumed, but completion processing has not yet run |
| 16 | * so queuing new data is blocked. |
| 17 | * 4) Minimizing the amount of queued data is desirable. |
| 18 | * |
| 19 | * The goal of dql is to calculate the limit as the minimum number of objects |
| 20 | * needed to prevent starvation. |
| 21 | * |
| 22 | * The primary functions of dql are: |
| 23 | * dql_queued - called when objects are enqueued to record number of objects |
| 24 | * dql_avail - returns how many objects are available to be queued based |
| 25 | * on the object limit and how many objects are already enqueued |
| 26 | * dql_completed - called at completion time to indicate how many objects |
| 27 | * were retired from the queue |
| 28 | * |
| 29 | * The dql implementation does not implement any locking for the dql data |
| 30 | * structures, the higher layer should provide this. dql_queued should |
| 31 | * be serialized to prevent concurrent execution of the function; this |
| 32 | * is also true for dql_completed. However, dql_queued and dlq_completed can |
| 33 | * be executed concurrently (i.e. they can be protected by different locks). |
| 34 | */ |
| 35 | |
| 36 | #ifndef _LINUX_DQL_H |
| 37 | #define _LINUX_DQL_H |
| 38 | |
| 39 | #ifdef __KERNEL__ |
| 40 | |
| 41 | #include <linux/bitops.h> |
| 42 | #include <asm/bug.h> |
| 43 | |
| 44 | #define DQL_HIST_LEN 4 |
| 45 | #define DQL_HIST_ENT(dql, idx) ((dql)->history[(idx) % DQL_HIST_LEN]) |
| 46 | |
| 47 | struct dql { |
| 48 | /* Fields accessed in enqueue path (dql_queued) */ |
| 49 | unsigned int num_queued; /* Total ever queued */ |
| 50 | unsigned int adj_limit; /* limit + num_completed */ |
| 51 | unsigned int last_obj_cnt; /* Count at last queuing */ |
| 52 | |
| 53 | /* Stall threshold (in jiffies), defined by user */ |
| 54 | unsigned short stall_thrs; |
| 55 | |
| 56 | unsigned long history_head; /* top 58 bits of jiffies */ |
| 57 | /* stall entries, a bit per entry */ |
| 58 | unsigned long history[DQL_HIST_LEN]; |
| 59 | |
| 60 | /* Fields accessed only by completion path (dql_completed) */ |
| 61 | |
| 62 | unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */ |
| 63 | unsigned int num_completed; /* Total ever completed */ |
| 64 | |
| 65 | unsigned int prev_ovlimit; /* Previous over limit */ |
| 66 | unsigned int prev_num_queued; /* Previous queue total */ |
| 67 | unsigned int prev_last_obj_cnt; /* Previous queuing cnt */ |
| 68 | |
| 69 | unsigned int lowest_slack; /* Lowest slack found */ |
| 70 | unsigned long slack_start_time; /* Time slacks seen */ |
| 71 | |
| 72 | /* Configuration */ |
| 73 | unsigned int max_limit; /* Max limit */ |
| 74 | unsigned int min_limit; /* Minimum limit */ |
| 75 | unsigned int slack_hold_time; /* Time to measure slack */ |
| 76 | |
| 77 | /* Longest stall detected, reported to user */ |
| 78 | unsigned short stall_max; |
| 79 | unsigned long last_reap; /* Last reap (in jiffies) */ |
| 80 | unsigned long stall_cnt; /* Number of stalls */ |
| 81 | }; |
| 82 | |
| 83 | /* Set some static maximums */ |
| 84 | #define DQL_MAX_OBJECT (UINT_MAX / 16) |
| 85 | #define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT) |
| 86 | |
| 87 | /* Populate the bitmap to be processed later in dql_check_stall() */ |
| 88 | static inline void dql_queue_stall(struct dql *dql) |
| 89 | { |
| 90 | unsigned long map, now, now_hi, i; |
| 91 | |
| 92 | now = jiffies; |
| 93 | now_hi = now / BITS_PER_LONG; |
| 94 | |
| 95 | /* The following code set a bit in the ring buffer, where each |
| 96 | * bit trackes time the packet was queued. The dql->history buffer |
| 97 | * tracks DQL_HIST_LEN * BITS_PER_LONG time (jiffies) slot |
| 98 | */ |
| 99 | if (unlikely(now_hi != dql->history_head)) { |
| 100 | /* About to reuse slots, clear them */ |
| 101 | for (i = 0; i < DQL_HIST_LEN; i++) { |
| 102 | /* Multiplication masks high bits */ |
| 103 | if (now_hi * BITS_PER_LONG == |
| 104 | (dql->history_head + i) * BITS_PER_LONG) |
| 105 | break; |
| 106 | DQL_HIST_ENT(dql, dql->history_head + i + 1) = 0; |
| 107 | } |
| 108 | /* pairs with smp_rmb() in dql_check_stall() */ |
| 109 | smp_wmb(); |
| 110 | WRITE_ONCE(dql->history_head, now_hi); |
| 111 | } |
| 112 | |
| 113 | /* __set_bit() does not guarantee WRITE_ONCE() semantics */ |
| 114 | map = DQL_HIST_ENT(dql, now_hi); |
| 115 | |
| 116 | /* Populate the history with an entry (bit) per queued */ |
| 117 | if (!(map & BIT_MASK(now))) |
| 118 | WRITE_ONCE(DQL_HIST_ENT(dql, now_hi), map | BIT_MASK(now)); |
| 119 | } |
| 120 | |
| 121 | /* |
| 122 | * Record number of objects queued. Assumes that caller has already checked |
| 123 | * availability in the queue with dql_avail. |
| 124 | */ |
| 125 | static inline void dql_queued(struct dql *dql, unsigned int count) |
| 126 | { |
| 127 | if (WARN_ON_ONCE(count > DQL_MAX_OBJECT)) |
| 128 | return; |
| 129 | |
| 130 | dql->last_obj_cnt = count; |
| 131 | |
| 132 | /* We want to force a write first, so that cpu do not attempt |
| 133 | * to get cache line containing last_obj_cnt, num_queued, adj_limit |
| 134 | * in Shared state, but directly does a Request For Ownership |
| 135 | * It is only a hint, we use barrier() only. |
| 136 | */ |
| 137 | barrier(); |
| 138 | |
| 139 | dql->num_queued += count; |
| 140 | |
| 141 | /* Only populate stall information if the threshold is set */ |
| 142 | if (READ_ONCE(dql->stall_thrs)) |
| 143 | dql_queue_stall(dql); |
| 144 | } |
| 145 | |
| 146 | /* Returns how many objects can be queued, < 0 indicates over limit. */ |
| 147 | static inline int dql_avail(const struct dql *dql) |
| 148 | { |
| 149 | return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued); |
| 150 | } |
| 151 | |
| 152 | /* Record number of completed objects and recalculate the limit. */ |
| 153 | void dql_completed(struct dql *dql, unsigned int count); |
| 154 | |
| 155 | /* Reset dql state */ |
| 156 | void dql_reset(struct dql *dql); |
| 157 | |
| 158 | /* Initialize dql state */ |
| 159 | void dql_init(struct dql *dql, unsigned int hold_time); |
| 160 | |
| 161 | #endif /* _KERNEL_ */ |
| 162 | |
| 163 | #endif /* _LINUX_DQL_H */ |