include/linux/dynamic_queue_limits.h

   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
  47 struct 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         /* Stall threshold (in jiffies), defined by user */
  54         unsigned short  stall_thrs;
  55
  56         unsigned long   history_head;           /* top 58 bits of jiffies */
  57         /* stall entries, a bit per entry */
  58         unsigned long   history[DQL_HIST_LEN];
  59
  60         /* Fields accessed only by completion path (dql_completed) */
  61
  62         unsigned int    limit ____cacheline_aligned_in_smp; /* Current limit */
  63         unsigned int    num_completed;          /* Total ever completed */
  64
  65         unsigned int    prev_ovlimit;           /* Previous over limit */
  66         unsigned int    prev_num_queued;        /* Previous queue total */
  67         unsigned int    prev_last_obj_cnt;      /* Previous queuing cnt */
  68
  69         unsigned int    lowest_slack;           /* Lowest slack found */
  70         unsigned long   slack_start_time;       /* Time slacks seen */
  71
  72         /* Configuration */
  73         unsigned int    max_limit;              /* Max limit */
  74         unsigned int    min_limit;              /* Minimum limit */
  75         unsigned int    slack_hold_time;        /* Time to measure slack */
  76
  77         /* Longest stall detected, reported to user */
  78         unsigned short  stall_max;
  79         unsigned long   last_reap;              /* Last reap (in jiffies) */
  80         unsigned long   stall_cnt;              /* Number of stalls */
  81 };
  82
  83 /* Set some static maximums */
  84 #define DQL_MAX_OBJECT (UINT_MAX / 16)
  85 #define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
  86
  87 /* Populate the bitmap to be processed later in dql_check_stall() */
  88 static inline void dql_queue_stall(struct dql *dql)
  89 {
  90         unsigned long map, now, now_hi, i;
  91
  92         now = jiffies;
  93         now_hi = now / BITS_PER_LONG;
  94
  95         /* The following code set a bit in the ring buffer, where each
  96          * bit trackes time the packet was queued. The dql->history buffer
  97          * tracks DQL_HIST_LEN * BITS_PER_LONG time (jiffies) slot
  98          */
  99         if (unlikely(now_hi != dql->history_head)) {
 100                 /* About to reuse slots, clear them */
 101                 for (i = 0; i < DQL_HIST_LEN; i++) {
 102                         /* Multiplication masks high bits */
 103                         if (now_hi * BITS_PER_LONG ==
 104                             (dql->history_head + i) * BITS_PER_LONG)
 105                                 break;
 106                         DQL_HIST_ENT(dql, dql->history_head + i + 1) = 0;
 107                 }
 108                 /* pairs with smp_rmb() in dql_check_stall() */
 109                 smp_wmb();
 110                 WRITE_ONCE(dql->history_head, now_hi);
 111         }
 112
 113         /* __set_bit() does not guarantee WRITE_ONCE() semantics */
 114         map = DQL_HIST_ENT(dql, now_hi);
 115
 116         /* Populate the history with an entry (bit) per queued */
 117         if (!(map & BIT_MASK(now)))
 118                 WRITE_ONCE(DQL_HIST_ENT(dql, now_hi), map | BIT_MASK(now));
 119 }
 120
 121 /*
 122  * Record number of objects queued. Assumes that caller has already checked
 123  * availability in the queue with dql_avail.
 124  */
 125 static inline void dql_queued(struct dql *dql, unsigned int count)
 126 {
 127         if (WARN_ON_ONCE(count > DQL_MAX_OBJECT))
 128                 return;
 129
 130         WRITE_ONCE(dql->last_obj_cnt, count);
 131
 132         /* We want to force a write first, so that cpu do not attempt
 133          * to get cache line containing last_obj_cnt, num_queued, adj_limit
 134          * in Shared state, but directly does a Request For Ownership
 135          * It is only a hint, we use barrier() only.
 136          */
 137         barrier();
 138
 139         dql->num_queued += count;
 140
 141         /* Only populate stall information if the threshold is set */
 142         if (READ_ONCE(dql->stall_thrs))
 143                 dql_queue_stall(dql);
 144 }
 145
 146 /* Returns how many objects can be queued, < 0 indicates over limit. */
 147 static inline int dql_avail(const struct dql *dql)
 148 {
 149         return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued);
 150 }
 151
 152 /* Record number of completed objects and recalculate the limit. */
 153 void dql_completed(struct dql *dql, unsigned int count);
 154
 155 /* Reset dql state */
 156 void dql_reset(struct dql *dql);
 157
 158 /* Initialize dql state */
 159 void dql_init(struct dql *dql, unsigned int hold_time);
 160
 161 #endif /* _KERNEL_ */
 162
 163 #endif /* _LINUX_DQL_H */