154 lines
5.2 KiB
C
154 lines
5.2 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* Dynamic queue limits (dql) - Definitions
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*
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* Copyright (c) 2011, Tom Herbert <therbert@google.com>
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*
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* This header file contains the definitions for dynamic queue limits (dql).
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* dql would be used in conjunction with a producer/consumer type queue
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* (possibly a HW queue). Such a queue would have these general properties:
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*
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* 1) Objects are queued up to some limit specified as number of objects.
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* 2) Periodically a completion process executes which retires consumed
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* objects.
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* 3) Starvation occurs when limit has been reached, all queued data has
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* actually been consumed, but completion processing has not yet run
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* so queuing new data is blocked.
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* 4) Minimizing the amount of queued data is desirable.
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*
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* The goal of dql is to calculate the limit as the minimum number of objects
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* needed to prevent starvation.
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*
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* The primary functions of dql are:
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* dql_queued - called when objects are enqueued to record number of objects
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* dql_avail - returns how many objects are available to be queued based
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* on the object limit and how many objects are already enqueued
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* dql_completed - called at completion time to indicate how many objects
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* were retired from the queue
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*
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* The dql implementation does not implement any locking for the dql data
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* structures, the higher layer should provide this. dql_queued should
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* be serialized to prevent concurrent execution of the function; this
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* is also true for dql_completed. However, dql_queued and dlq_completed can
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* be executed concurrently (i.e. they can be protected by different locks).
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*/
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#ifndef _LINUX_DQL_H
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#define _LINUX_DQL_H
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#ifdef __KERNEL__
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#include <linux/bitops.h>
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#include <asm/bug.h>
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#define DQL_HIST_LEN 4
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#define DQL_HIST_ENT(dql, idx) ((dql)->history[(idx) % DQL_HIST_LEN])
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struct dql {
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/* Fields accessed in enqueue path (dql_queued) */
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unsigned int num_queued; /* Total ever queued */
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unsigned int adj_limit; /* limit + num_completed */
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unsigned int last_obj_cnt; /* Count at last queuing */
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unsigned long history_head; /* top 58 bits of jiffies */
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/* stall entries, a bit per entry */
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unsigned long history[DQL_HIST_LEN];
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/* Fields accessed only by completion path (dql_completed) */
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unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */
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unsigned int num_completed; /* Total ever completed */
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unsigned int prev_ovlimit; /* Previous over limit */
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unsigned int prev_num_queued; /* Previous queue total */
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unsigned int prev_last_obj_cnt; /* Previous queuing cnt */
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unsigned int lowest_slack; /* Lowest slack found */
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unsigned long slack_start_time; /* Time slacks seen */
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/* Configuration */
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unsigned int max_limit; /* Max limit */
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unsigned int min_limit; /* Minimum limit */
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unsigned int slack_hold_time; /* Time to measure slack */
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/* Stall threshold (in jiffies), defined by user */
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unsigned short stall_thrs;
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/* Longest stall detected, reported to user */
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unsigned short stall_max;
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unsigned long last_reap; /* Last reap (in jiffies) */
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unsigned long stall_cnt; /* Number of stalls */
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};
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/* Set some static maximums */
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#define DQL_MAX_OBJECT (UINT_MAX / 16)
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#define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
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/*
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* Record number of objects queued. Assumes that caller has already checked
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* availability in the queue with dql_avail.
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*/
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static inline void dql_queued(struct dql *dql, unsigned int count)
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{
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unsigned long map, now, now_hi, i;
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BUG_ON(count > DQL_MAX_OBJECT);
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dql->last_obj_cnt = count;
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/* We want to force a write first, so that cpu do not attempt
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* to get cache line containing last_obj_cnt, num_queued, adj_limit
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* in Shared state, but directly does a Request For Ownership
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* It is only a hint, we use barrier() only.
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*/
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barrier();
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dql->num_queued += count;
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now = jiffies;
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now_hi = now / BITS_PER_LONG;
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/* The following code set a bit in the ring buffer, where each
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* bit trackes time the packet was queued. The dql->history buffer
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* tracks DQL_HIST_LEN * BITS_PER_LONG time (jiffies) slot
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*/
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if (unlikely(now_hi != dql->history_head)) {
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/* About to reuse slots, clear them */
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for (i = 0; i < DQL_HIST_LEN; i++) {
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/* Multiplication masks high bits */
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if (now_hi * BITS_PER_LONG ==
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(dql->history_head + i) * BITS_PER_LONG)
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break;
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DQL_HIST_ENT(dql, dql->history_head + i + 1) = 0;
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}
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/* pairs with smp_rmb() in dql_check_stall() */
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smp_wmb();
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WRITE_ONCE(dql->history_head, now_hi);
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}
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/* __set_bit() does not guarantee WRITE_ONCE() semantics */
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map = DQL_HIST_ENT(dql, now_hi);
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/* Populate the history with an entry (bit) per queued */
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if (!(map & BIT_MASK(now)))
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WRITE_ONCE(DQL_HIST_ENT(dql, now_hi), map | BIT_MASK(now));
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}
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/* Returns how many objects can be queued, < 0 indicates over limit. */
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static inline int dql_avail(const struct dql *dql)
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{
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return READ_ONCE(dql->adj_limit) - READ_ONCE(dql->num_queued);
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}
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/* Record number of completed objects and recalculate the limit. */
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void dql_completed(struct dql *dql, unsigned int count);
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/* Reset dql state */
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void dql_reset(struct dql *dql);
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/* Initialize dql state */
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void dql_init(struct dql *dql, unsigned int hold_time);
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#endif /* _KERNEL_ */
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#endif /* _LINUX_DQL_H */
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