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
 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	/* 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() */
 88static 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 */
125static 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. */
147static 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. */
153void dql_completed(struct dql *dql, unsigned int count);
154
155/* Reset dql state */
156void dql_reset(struct dql *dql);
157
158/* Initialize dql state */
159void dql_init(struct dql *dql, unsigned int hold_time);
160
161#endif /* _KERNEL_ */
162
163#endif /* _LINUX_DQL_H */