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/* SPDX-License-Identifier: GPL-2.0 */
* Dynamic queue limits (dql) - Definitions
* Copyright (c) 2011, Tom Herbert <>
* This header file contains the definitions for dynamic queue limits (dql).
* dql would be used in conjunction with a producer/consumer type queue
* (possibly a HW queue). Such a queue would have these general properties:
* 1) Objects are queued up to some limit specified as number of objects.
* 2) Periodically a completion process executes which retires consumed
* objects.
* 3) Starvation occurs when limit has been reached, all queued data has
* actually been consumed, but completion processing has not yet run
* so queuing new data is blocked.
* 4) Minimizing the amount of queued data is desirable.
* The goal of dql is to calculate the limit as the minimum number of objects
* needed to prevent starvation.
* The primary functions of dql are:
* dql_queued - called when objects are enqueued to record number of objects
* dql_avail - returns how many objects are available to be queued based
* on the object limit and how many objects are already enqueued
* dql_completed - called at completion time to indicate how many objects
* were retired from the queue
* The dql implementation does not implement any locking for the dql data
* structures, the higher layer should provide this. dql_queued should
* be serialized to prevent concurrent execution of the function; this
* is also true for dql_completed. However, dql_queued and dlq_completed can
* be executed concurrently (i.e. they can be protected by different locks).
#ifndef _LINUX_DQL_H
#define _LINUX_DQL_H
#ifdef __KERNEL__
#include <asm/bug.h>
struct dql {
/* Fields accessed in enqueue path (dql_queued) */
unsigned int num_queued; /* Total ever queued */
unsigned int adj_limit; /* limit + num_completed */
unsigned int last_obj_cnt; /* Count at last queuing */
/* Fields accessed only by completion path (dql_completed) */
unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */
unsigned int num_completed; /* Total ever completed */
unsigned int prev_ovlimit; /* Previous over limit */
unsigned int prev_num_queued; /* Previous queue total */
unsigned int prev_last_obj_cnt; /* Previous queuing cnt */
unsigned int lowest_slack; /* Lowest slack found */
unsigned long slack_start_time; /* Time slacks seen */
/* Configuration */
unsigned int max_limit; /* Max limit */
unsigned int min_limit; /* Minimum limit */
unsigned int slack_hold_time; /* Time to measure slack */
/* Set some static maximums */
#define DQL_MAX_OBJECT (UINT_MAX / 16)
* Record number of objects queued. Assumes that caller has already checked
* availability in the queue with dql_avail.
static inline void dql_queued(struct dql *dql, unsigned int count)
dql->last_obj_cnt = count;
/* We want to force a write first, so that cpu do not attempt
* to get cache line containing last_obj_cnt, num_queued, adj_limit
* in Shared state, but directly does a Request For Ownership
* It is only a hint, we use barrier() only.
dql->num_queued += count;
/* Returns how many objects can be queued, < 0 indicates over limit. */
static inline int dql_avail(const struct dql *dql)
return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued);
/* Record number of completed objects and recalculate the limit. */
void dql_completed(struct dql *dql, unsigned int count);
/* Reset dql state */
void dql_reset(struct dql *dql);
/* Initialize dql state */
void dql_init(struct dql *dql, unsigned int hold_time);
#endif /* _KERNEL_ */
#endif /* _LINUX_DQL_H */