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