path: root/block/bfq-wf2q.c

/*
 * Hierarchical Budget Worst-case Fair Weighted Fair Queueing
 * (B-WF2Q+): hierarchical scheduling algorithm by which the BFQ I/O
 * scheduler schedules generic entities. The latter can represent
 * either single bfq queues (associated with processes) or groups of
 * bfq queues (associated with cgroups).
 *
 *  This program is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU General Public License as
 *  published by the Free Software Foundation; either version 2 of the
 *  License, or (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  General Public License for more details.
 */
#include "bfq-iosched.h"

/**
 * bfq_gt - compare two timestamps.
 * @a: first ts.
 * @b: second ts.
 *
 * Return @a > @b, dealing with wrapping correctly.
 */
static int bfq_gt(u64 a, u64 b)
{
	return (s64)(a - b) > 0;
}

static struct bfq_entity *bfq_root_active_entity(struct rb_root *tree)
{
	struct rb_node *node = tree->rb_node;

	return rb_entry(node, struct bfq_entity, rb_node);
}

static unsigned int bfq_class_idx(struct bfq_entity *entity)
{
	struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity);

	return bfqq ? bfqq->ioprio_class - 1 :
		BFQ_DEFAULT_GRP_CLASS - 1;
}

static struct bfq_entity *bfq_lookup_next_entity(struct bfq_sched_data *sd);

static bool bfq_update_parent_budget(struct bfq_entity *next_in_service);

/**
 * bfq_update_next_in_service - update sd->next_in_service
 * @sd: sched_data for which to perform the update.
 * @new_entity: if not NULL, pointer to the entity whose activation,
 *		requeueing or repositionig triggered the invocation of
 *		this function.
 *
 * This function is called to update sd->next_in_service, which, in
 * its turn, may change as a consequence of the insertion or
 * extraction of an entity into/from one of the active trees of
 * sd. These insertions/extractions occur as a consequence of
 * activations/deactivations of entities, with some activations being
 * 'true' activations, and other activations being requeueings (i.e.,
 * implementing the second, requeueing phase of the mechanism used to
 * reposition an entity in its active tree; see comments on
 * __bfq_activate_entity and __bfq_requeue_entity for details). In
 * both the last two activation sub-cases, new_entity points to the
 * just activated or requeued entity.
 *
 * Returns true if sd->next_in_service changes in such a way that
 * entity->parent may become the next_in_service for its parent
 * entity.
 */
static bool bfq_update_next_in_service(struct bfq_sched_data *sd,
				       struct bfq_entity *new_entity)
{
	struct bfq_entity *next_in_service = sd->next_in_service;
	bool parent_sched_may_change = false;

	/*
	 * If this update is triggered by the activation, requeueing
	 * or repositiong of an entity that does not coincide with
	 * sd->next_in_service, then a full lookup in the active tree
	 * can be avoided. In fact, it is enough to check whether the
	 * just-modified entity has a higher priority than
	 * sd->next_in_service, or, even if it has the same priority
	 * as sd->next_in_service, is eligible and has a lower virtual
	 * finish time than sd->next_in_service. If this compound
	 * condition holds, then the new entity becomes the new
	 * next_in_service. Otherwise no change is needed.
	 */
	if (new_entity && new_entity != sd->next_in_service) {
		/*
		 * Flag used to decide whether to replace
		 * sd->next_in_service with new_entity. Tentatively
		 * set to true, and left as true if
		 * sd->next_in_service is NULL.
		 */
		bool replace_next = true;

		/*
		 * If there is already a next_in_service candidate
		 * entity, then compare class priorities or timestamps
		 * to decide whether to replace sd->service_tree with
		 * new_entity.
		 */
		if (next_in_service) {
			unsigned int new_entity_class_idx =
				bfq_class_idx(new_entity);
			struct bfq_service_tree *st =
				sd->service_tr