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Release 4.12 include/linux/dynamic_queue_limits.h

Directory: include/linux
 * 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__

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) { BUG_ON(count > DQL_MAX_OBJECT); 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. */ barrier(); dql->num_queued += count; }


Tom Herbert3179.49%150.00%
Eric Dumazet820.51%150.00%

/* Returns how many objects can be queued, < 0 indicates over limit. */
static inline int dql_avail(const struct dql *dql) { return ACCESS_ONCE(dql->adj_limit) - ACCESS_ONCE(dql->num_queued); }


Tom Herbert2278.57%150.00%
Eric Dumazet621.43%150.00%

/* 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 */ int dql_init(struct dql *dql, unsigned hold_time); #endif /* _KERNEL_ */ #endif /* _LINUX_DQL_H */

Overall Contributors

Tom Herbert19193.17%150.00%
Eric Dumazet146.83%150.00%
Directory: include/linux
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