kernel / pub / scm / linux / kernel / git / gregkh / tty / 60fce36afa9c77c7ccbf980c4f670f3be3651fce / . / lib / dynamic_queue_limits.c

// SPDX-License-Identifier: GPL-2.0 | |

/* | |

* Dynamic byte queue limits. See include/linux/dynamic_queue_limits.h | |

* | |

* Copyright (c) 2011, Tom Herbert <therbert@google.com> | |

*/ | |

#include <linux/types.h> | |

#include <linux/kernel.h> | |

#include <linux/jiffies.h> | |

#include <linux/dynamic_queue_limits.h> | |

#include <linux/compiler.h> | |

#include <linux/export.h> | |

#define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0) | |

#define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0) | |

/* Records completed count and recalculates the queue limit */ | |

void dql_completed(struct dql *dql, unsigned int count) | |

{ | |

unsigned int inprogress, prev_inprogress, limit; | |

unsigned int ovlimit, completed, num_queued; | |

bool all_prev_completed; | |

num_queued = READ_ONCE(dql->num_queued); | |

/* Can't complete more than what's in queue */ | |

BUG_ON(count > num_queued - dql->num_completed); | |

completed = dql->num_completed + count; | |

limit = dql->limit; | |

ovlimit = POSDIFF(num_queued - dql->num_completed, limit); | |

inprogress = num_queued - completed; | |

prev_inprogress = dql->prev_num_queued - dql->num_completed; | |

all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued); | |

if ((ovlimit && !inprogress) || | |

(dql->prev_ovlimit && all_prev_completed)) { | |

/* | |

* Queue considered starved if: | |

* - The queue was over-limit in the last interval, | |

* and there is no more data in the queue. | |

* OR | |

* - The queue was over-limit in the previous interval and | |

* when enqueuing it was possible that all queued data | |

* had been consumed. This covers the case when queue | |

* may have becomes starved between completion processing | |

* running and next time enqueue was scheduled. | |

* | |

* When queue is starved increase the limit by the amount | |

* of bytes both sent and completed in the last interval, | |

* plus any previous over-limit. | |

*/ | |

limit += POSDIFF(completed, dql->prev_num_queued) + | |

dql->prev_ovlimit; | |

dql->slack_start_time = jiffies; | |

dql->lowest_slack = UINT_MAX; | |

} else if (inprogress && prev_inprogress && !all_prev_completed) { | |

/* | |

* Queue was not starved, check if the limit can be decreased. | |

* A decrease is only considered if the queue has been busy in | |

* the whole interval (the check above). | |

* | |

* If there is slack, the amount of execess data queued above | |

* the the amount needed to prevent starvation, the queue limit | |

* can be decreased. To avoid hysteresis we consider the | |

* minimum amount of slack found over several iterations of the | |

* completion routine. | |

*/ | |

unsigned int slack, slack_last_objs; | |

/* | |

* Slack is the maximum of | |

* - The queue limit plus previous over-limit minus twice | |

* the number of objects completed. Note that two times | |

* number of completed bytes is a basis for an upper bound | |

* of the limit. | |

* - Portion of objects in the last queuing operation that | |

* was not part of non-zero previous over-limit. That is | |

* "round down" by non-overlimit portion of the last | |

* queueing operation. | |

*/ | |

slack = POSDIFF(limit + dql->prev_ovlimit, | |

2 * (completed - dql->num_completed)); | |

slack_last_objs = dql->prev_ovlimit ? | |

POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0; | |

slack = max(slack, slack_last_objs); | |

if (slack < dql->lowest_slack) | |

dql->lowest_slack = slack; | |

if (time_after(jiffies, | |

dql->slack_start_time + dql->slack_hold_time)) { | |

limit = POSDIFF(limit, dql->lowest_slack); | |

dql->slack_start_time = jiffies; | |

dql->lowest_slack = UINT_MAX; | |

} | |

} | |

/* Enforce bounds on limit */ | |

limit = clamp(limit, dql->min_limit, dql->max_limit); | |

if (limit != dql->limit) { | |

dql->limit = limit; | |

ovlimit = 0; | |

} | |

dql->adj_limit = limit + completed; | |

dql->prev_ovlimit = ovlimit; | |

dql->prev_last_obj_cnt = dql->last_obj_cnt; | |

dql->num_completed = completed; | |

dql->prev_num_queued = num_queued; | |

} | |

EXPORT_SYMBOL(dql_completed); | |

void dql_reset(struct dql *dql) | |

{ | |

/* Reset all dynamic values */ | |

dql->limit = 0; | |

dql->num_queued = 0; | |

dql->num_completed = 0; | |

dql->last_obj_cnt = 0; | |

dql->prev_num_queued = 0; | |

dql->prev_last_obj_cnt = 0; | |

dql->prev_ovlimit = 0; | |

dql->lowest_slack = UINT_MAX; | |

dql->slack_start_time = jiffies; | |

} | |

EXPORT_SYMBOL(dql_reset); | |

void dql_init(struct dql *dql, unsigned int hold_time) | |

{ | |

dql->max_limit = DQL_MAX_LIMIT; | |

dql->min_limit = 0; | |

dql->slack_hold_time = hold_time; | |

dql_reset(dql); | |

} | |

EXPORT_SYMBOL(dql_init); |