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 47struct 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 unsigned long history_head; /* top 58 bits of jiffies */ 54 /* stall entries, a bit per entry */ 55 unsigned long history[DQL_HIST_LEN]; 56 57 /* Fields accessed only by completion path (dql_completed) */ 58 59 unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */ 60 unsigned int num_completed; /* Total ever completed */ 61 62 unsigned int prev_ovlimit; /* Previous over limit */ 63 unsigned int prev_num_queued; /* Previous queue total */ 64 unsigned int prev_last_obj_cnt; /* Previous queuing cnt */ 65 66 unsigned int lowest_slack; /* Lowest slack found */ 67 unsigned long slack_start_time; /* Time slacks seen */ 68 69 /* Configuration */ 70 unsigned int max_limit; /* Max limit */ 71 unsigned int min_limit; /* Minimum limit */ 72 unsigned int slack_hold_time; /* Time to measure slack */ 73 74 /* Stall threshold (in jiffies), defined by user */ 75 unsigned short stall_thrs; 76 /* Longest stall detected, reported to user */ 77 unsigned short stall_max; 78 unsigned long last_reap; /* Last reap (in jiffies) */ 79 unsigned long stall_cnt; /* Number of stalls */ 80}; 81 82/* Set some static maximums */ 83#define DQL_MAX_OBJECT (UINT_MAX / 16) 84#define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT) 85 86/* 87 * Record number of objects queued. Assumes that caller has already checked 88 * availability in the queue with dql_avail. 89 */ 90static inline void dql_queued(struct dql *dql, unsigned int count) 91{ 92 unsigned long map, now, now_hi, i; 93 94 BUG_ON(count > DQL_MAX_OBJECT); 95 96 dql->last_obj_cnt = count; 97 98 /* We want to force a write first, so that cpu do not attempt 99 * to get cache line containing last_obj_cnt, num_queued, adj_limit 100 * in Shared state, but directly does a Request For Ownership 101 * It is only a hint, we use barrier() only. 102 */ 103 barrier(); 104 105 dql->num_queued += count; 106 107 now = jiffies; 108 now_hi = now / BITS_PER_LONG; 109 110 /* The following code set a bit in the ring buffer, where each 111 * bit trackes time the packet was queued. The dql->history buffer 112 * tracks DQL_HIST_LEN * BITS_PER_LONG time (jiffies) slot 113 */ 114 if (unlikely(now_hi != dql->history_head)) { 115 /* About to reuse slots, clear them */ 116 for (i = 0; i < DQL_HIST_LEN; i++) { 117 /* Multiplication masks high bits */ 118 if (now_hi * BITS_PER_LONG == 119 (dql->history_head + i) * BITS_PER_LONG) 120 break; 121 DQL_HIST_ENT(dql, dql->history_head + i + 1) = 0; 122 } 123 /* pairs with smp_rmb() in dql_check_stall() */ 124 smp_wmb(); 125 WRITE_ONCE(dql->history_head, now_hi); 126 } 127 128 /* __set_bit() does not guarantee WRITE_ONCE() semantics */ 129 map = DQL_HIST_ENT(dql, now_hi); 130 131 /* Populate the history with an entry (bit) per queued */ 132 if (!(map & BIT_MASK(now))) 133 WRITE_ONCE(DQL_HIST_ENT(dql, now_hi), map | BIT_MASK(now)); 134} 135 136/* Returns how many objects can be queued, < 0 indicates over limit. */ 137static inline int dql_avail(const struct dql *dql) 138{ 139 return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued); 140} 141 142/* Record number of completed objects and recalculate the limit. */ 143void dql_completed(struct dql *dql, unsigned int count); 144 145/* Reset dql state */ 146void dql_reset(struct dql *dql); 147 148/* Initialize dql state */ 149void dql_init(struct dql *dql, unsigned int hold_time); 150 151#endif /* _KERNEL_ */ 152 153#endif /* _LINUX_DQL_H */ 154