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 struct dql { 42 /* Fields accessed in enqueue path (dql_queued) */ 43 unsigned int num_queued; /* Total ever queued */ 44 unsigned int adj_limit; /* limit + num_completed */ 45 unsigned int last_obj_cnt; /* Count at last queuing */ 46 47 /* Fields accessed only by completion path (dql_completed) */ 48 49 unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */ 50 unsigned int num_completed; /* Total ever completed */ 51 52 unsigned int prev_ovlimit; /* Previous over limit */ 53 unsigned int prev_num_queued; /* Previous queue total */ 54 unsigned int prev_last_obj_cnt; /* Previous queuing cnt */ 55 56 unsigned int lowest_slack; /* Lowest slack found */ 57 unsigned long slack_start_time; /* Time slacks seen */ 58 59 /* Configuration */ 60 unsigned int max_limit; /* Max limit */ 61 unsigned int min_limit; /* Minimum limit */ 62 unsigned int slack_hold_time; /* Time to measure slack */ 63 }; 64 65 /* Set some static maximums */ 66 #define DQL_MAX_OBJECT (UINT_MAX / 16) 67 #define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT) 68 69 /* 70 * Record number of objects queued. Assumes that caller has already checked 71 * availability in the queue with dql_avail. 72 */ 73 static inline void dql_queued(struct dql *dql, unsigned int count) 74 { 75 BUG_ON(count > DQL_MAX_OBJECT); 76 77 dql->last_obj_cnt = count; 78 79 /* We want to force a write first, so that cpu do not attempt 80 * to get cache line containing last_obj_cnt, num_queued, adj_limit 81 * in Shared state, but directly does a Request For Ownership 82 * It is only a hint, we use barrier() only. 83 */ 84 barrier(); 85 86 dql->num_queued += count; 87 } 88 89 /* Returns how many objects can be queued, < 0 indicates over limit. */ 90 static inline int dql_avail(const struct dql *dql) 91 { 92 return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued); 93 } 94 95 /* Record number of completed objects and recalculate the limit. */ 96 void dql_completed(struct dql *dql, unsigned int count); 97 98 /* Reset dql state */ 99 void dql_reset(struct dql *dql); 100 101 /* Initialize dql state */ 102 void dql_init(struct dql *dql, unsigned int hold_time); 103 104 #endif /* _KERNEL_ */ 105 106 #endif /* _LINUX_DQL_H */ 107