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/* SPDX-License-Identifier: GPL-2.0 */
/*
* Dynamic queue limits (dql) - Definitions
*
* Copyright (c) 2011, Tom Herbert <therbert@google.com>
*
* 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 <linux/bitops.h>
#include <asm/bug.h>
#define DQL_HIST_LEN 4
#define DQL_HIST_ENT(dql, idx) ((dql)->history[(idx) % DQL_HIST_LEN])
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 */
unsigned long history_head; /* top 58 bits of jiffies */
/* stall entries, a bit per entry */
unsigned long history[DQL_HIST_LEN];
/* 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 */
/* Stall threshold (in jiffies), defined by user */
unsigned short stall_thrs;
/* Longest stall detected, reported to user */
unsigned short stall_max;
unsigned long last_reap; /* Last reap (in jiffies) */
unsigned long stall_cnt; /* Number of stalls */
};
/* Set some static maximums */
#define DQL_MAX_OBJECT (UINT_MAX / 16)
#define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
/*
* 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)
{
unsigned long map, now, now_hi, i;
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;
now = jiffies;
now_hi = now / BITS_PER_LONG;
/* The following code set a bit in the ring buffer, where each
* bit trackes time the packet was queued. The dql->history buffer
* tracks DQL_HIST_LEN * BITS_PER_LONG time (jiffies) slot
*/
if (unlikely(now_hi != dql->history_head)) {
/* About to reuse slots, clear them */
for (i = 0; i < DQL_HIST_LEN; i++) {
/* Multiplication masks high bits */
if (now_hi * BITS_PER_LONG ==
(dql->history_head + i) * BITS_PER_LONG)
break;
DQL_HIST_ENT(dql, dql->history_head + i + 1) = 0;
}
/* pairs with smp_rmb() in dql_check_stall() */
smp_wmb();
WRITE_ONCE(dql->history_head, now_hi);
}
/* __set_bit() does not guarantee WRITE_ONCE() semantics */
map = DQL_HIST_ENT(dql, now_hi);
/* Populate the history with an entry (bit) per queued */
if (!(map & BIT_MASK(now)))
WRITE_ONCE(DQL_HIST_ENT(dql, now_hi), map | BIT_MASK(now));
}
/* 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 */
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