Linux Kernel  3.7.1
 All Data Structures Namespaces Files Functions Variables Typedefs Enumerations Enumerator Macros Groups Pages
dynamic_queue_limits.h
Go to the documentation of this file.
1 /*
2  * Dynamic queue limits (dql) - Definitions
3  *
4  * Copyright (c) 2011, Tom Herbert <[email protected]>
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 
40 struct 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  */
72 static inline void dql_queued(struct dql *dql, unsigned int count)
73 {
74  BUG_ON(count > DQL_MAX_OBJECT);
75 
76  dql->num_queued += count;
77  dql->last_obj_cnt = count;
78 }
79 
80 /* Returns how many objects can be queued, < 0 indicates over limit. */
81 static inline int dql_avail(const struct dql *dql)
82 {
83  return dql->adj_limit - dql->num_queued;
84 }
85 
86 /* Record number of completed objects and recalculate the limit. */
87 void dql_completed(struct dql *dql, unsigned int count);
88 
89 /* Reset dql state */
90 void dql_reset(struct dql *dql);
91 
92 /* Initialize dql state */
93 int dql_init(struct dql *dql, unsigned hold_time);
94 
95 #endif /* _KERNEL_ */
96 
97 #endif /* _LINUX_DQL_H */
Generated on Thu Jan 10 2013 14:51:20 for Linux Kernel by   doxygen 1.8.2