1/* 2 * Dynamic queue limits (dql) - Definitions 3 * 4 * Copyright (c) 2011, Tom Herbert <therbert@google.com> 5 * 6 * This header file contains the definitions for dynamic queue limits (dql). 7 * dql would be used in conjunction with a producer/consumer type queue 8 * (possibly a HW queue). Such a queue would have these general properties: 9 * 10 * 1) Objects are queued up to some limit specified as number of objects. 11 * 2) Periodically a completion process executes which retires consumed 12 * objects. 13 * 3) Starvation occurs when limit has been reached, all queued data has 14 * actually been consumed, but completion processing has not yet run 15 * so queuing new data is blocked. 16 * 4) Minimizing the amount of queued data is desirable. 17 * 18 * The goal of dql is to calculate the limit as the minimum number of objects 19 * needed to prevent starvation. 20 * 21 * The primary functions of dql are: 22 * dql_queued - called when objects are enqueued to record number of objects 23 * dql_avail - returns how many objects are available to be queued based 24 * on the object limit and how many objects are already enqueued 25 * dql_completed - called at completion time to indicate how many objects 26 * were retired from the queue 27 * 28 * The dql implementation does not implement any locking for the dql data 29 * structures, the higher layer should provide this. dql_queued should 30 * be serialized to prevent concurrent execution of the function; this 31 * is also true for dql_completed. However, dql_queued and dlq_completed can 32 * be executed concurrently (i.e. they can be protected by different locks). 33 */ 34 35#ifndef _LINUX_DQL_H 36#define _LINUX_DQL_H 37 38#ifdef __KERNEL__ 39 40struct dql { 41 /* Fields accessed in enqueue path (dql_queued) */ 42 unsigned int num_queued; /* Total ever queued */ 43 unsigned int adj_limit; /* limit + num_completed */ 44 unsigned int last_obj_cnt; /* Count at last queuing */ 45 46 /* Fields accessed only by completion path (dql_completed) */ 47 48 unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */ 49 unsigned int num_completed; /* Total ever completed */ 50 51 unsigned int prev_ovlimit; /* Previous over limit */ 52 unsigned int prev_num_queued; /* Previous queue total */ 53 unsigned int prev_last_obj_cnt; /* Previous queuing cnt */ 54 55 unsigned int lowest_slack; /* Lowest slack found */ 56 unsigned long slack_start_time; /* Time slacks seen */ 57 58 /* Configuration */ 59 unsigned int max_limit; /* Max limit */ 60 unsigned int min_limit; /* Minimum limit */ 61 unsigned int slack_hold_time; /* Time to measure slack */ 62}; 63 64/* Set some static maximums */ 65#define DQL_MAX_OBJECT (UINT_MAX / 16) 66#define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT) 67 68/* 69 * Record number of objects queued. Assumes that caller has already checked 70 * availability in the queue with dql_avail. 71 */ 72static inline void dql_queued(struct dql *dql, unsigned int count) 73{ 74 BUG_ON(count > DQL_MAX_OBJECT); 75 76 dql->last_obj_cnt = count; 77 78 /* We want to force a write first, so that cpu do not attempt 79 * to get cache line containing last_obj_cnt, num_queued, adj_limit 80 * in Shared state, but directly does a Request For Ownership 81 * It is only a hint, we use barrier() only. 82 */ 83 barrier(); 84 85 dql->num_queued += count; 86} 87 88/* Returns how many objects can be queued, < 0 indicates over limit. */ 89static inline int dql_avail(const struct dql *dql) 90{ 91 return ACCESS_ONCE(dql->adj_limit) - ACCESS_ONCE(dql->num_queued); 92} 93 94/* Record number of completed objects and recalculate the limit. */ 95void dql_completed(struct dql *dql, unsigned int count); 96 97/* Reset dql state */ 98void dql_reset(struct dql *dql); 99 100/* Initialize dql state */ 101int dql_init(struct dql *dql, unsigned hold_time); 102 103#endif /* _KERNEL_ */ 104 105#endif /* _LINUX_DQL_H */ 106