dynamic_queue_limits.h 3.7 KB

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