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() */ dql_queue_stall(struct dql * dql)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 */ dql_queued(struct dql * dql,unsigned int count)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. */ dql_avail(const struct dql * dql)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 */ 164