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![John Garry](/assets/img/avatar_default.png)
The tags used for an IO scheduler are currently per hctx. As such, when q->nr_hw_queues grows, so does the request queue total IO scheduler tag depth. This may cause problems for SCSI MQ HBAs whose total driver depth is fixed. Ming and Yanhui report higher CPU usage and lower throughput in scenarios where the fixed total driver tag depth is appreciably lower than the total scheduler tag depth: https://lore.kernel.org/linux-block/440dfcfc-1a2c-bd98-1161-cec4d78c6dfc@huawei.com/T/#mc0d6d4f95275a2743d1c8c3e4dc9ff6c9aa3a76b In that scenario, since the scheduler tag is got first, much contention is introduced since a driver tag may not be available after we have got the sched tag. Improve this scenario by introducing request queue-wide tags for when a tagset-wide sbitmap is used. The static sched requests are still allocated per hctx, as requests are initialised per hctx, as in blk_mq_init_request(..., hctx_idx, ...) -> set->ops->init_request(.., hctx_idx, ...). For simplicity of resizing the request queue sbitmap when updating the request queue depth, just init at the max possible size, so we don't need to deal with the possibly with swapping out a new sbitmap for old if we need to grow. Signed-off-by: John Garry <john.garry@huawei.com> Reviewed-by: Ming Lei <ming.lei@redhat.com> Link: https://lore.kernel.org/r/1620907258-30910-3-git-send-email-john.garry@huawei.com Signed-off-by: Jens Axboe <axboe@kernel.dk>
654 lines
18 KiB
C
654 lines
18 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Tag allocation using scalable bitmaps. Uses active queue tracking to support
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* fairer distribution of tags between multiple submitters when a shared tag map
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* is used.
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*
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* Copyright (C) 2013-2014 Jens Axboe
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*/
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/blk-mq.h>
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#include <linux/delay.h>
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#include "blk.h"
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#include "blk-mq.h"
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#include "blk-mq-sched.h"
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#include "blk-mq-tag.h"
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/*
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* If a previously inactive queue goes active, bump the active user count.
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* We need to do this before try to allocate driver tag, then even if fail
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* to get tag when first time, the other shared-tag users could reserve
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* budget for it.
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*/
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bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
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{
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if (blk_mq_is_sbitmap_shared(hctx->flags)) {
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struct request_queue *q = hctx->queue;
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struct blk_mq_tag_set *set = q->tag_set;
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if (!test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) &&
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!test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags))
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atomic_inc(&set->active_queues_shared_sbitmap);
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} else {
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if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
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!test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
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atomic_inc(&hctx->tags->active_queues);
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}
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return true;
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}
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/*
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* Wakeup all potentially sleeping on tags
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*/
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void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool include_reserve)
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{
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sbitmap_queue_wake_all(tags->bitmap_tags);
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if (include_reserve)
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sbitmap_queue_wake_all(tags->breserved_tags);
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}
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/*
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* If a previously busy queue goes inactive, potential waiters could now
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* be allowed to queue. Wake them up and check.
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*/
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void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
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{
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struct blk_mq_tags *tags = hctx->tags;
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struct request_queue *q = hctx->queue;
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struct blk_mq_tag_set *set = q->tag_set;
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if (blk_mq_is_sbitmap_shared(hctx->flags)) {
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if (!test_and_clear_bit(QUEUE_FLAG_HCTX_ACTIVE,
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&q->queue_flags))
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return;
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atomic_dec(&set->active_queues_shared_sbitmap);
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} else {
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if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
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return;
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atomic_dec(&tags->active_queues);
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}
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blk_mq_tag_wakeup_all(tags, false);
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}
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static int __blk_mq_get_tag(struct blk_mq_alloc_data *data,
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struct sbitmap_queue *bt)
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{
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if (!data->q->elevator && !(data->flags & BLK_MQ_REQ_RESERVED) &&
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!hctx_may_queue(data->hctx, bt))
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return BLK_MQ_NO_TAG;
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if (data->shallow_depth)
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return __sbitmap_queue_get_shallow(bt, data->shallow_depth);
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else
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return __sbitmap_queue_get(bt);
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}
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unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
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{
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struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
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struct sbitmap_queue *bt;
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struct sbq_wait_state *ws;
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DEFINE_SBQ_WAIT(wait);
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unsigned int tag_offset;
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int tag;
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if (data->flags & BLK_MQ_REQ_RESERVED) {
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if (unlikely(!tags->nr_reserved_tags)) {
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WARN_ON_ONCE(1);
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return BLK_MQ_NO_TAG;
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}
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bt = tags->breserved_tags;
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tag_offset = 0;
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} else {
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bt = tags->bitmap_tags;
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tag_offset = tags->nr_reserved_tags;
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}
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tag = __blk_mq_get_tag(data, bt);
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if (tag != BLK_MQ_NO_TAG)
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goto found_tag;
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if (data->flags & BLK_MQ_REQ_NOWAIT)
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return BLK_MQ_NO_TAG;
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ws = bt_wait_ptr(bt, data->hctx);
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do {
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struct sbitmap_queue *bt_prev;
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/*
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* We're out of tags on this hardware queue, kick any
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* pending IO submits before going to sleep waiting for
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* some to complete.
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*/
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blk_mq_run_hw_queue(data->hctx, false);
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/*
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* Retry tag allocation after running the hardware queue,
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* as running the queue may also have found completions.
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*/
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tag = __blk_mq_get_tag(data, bt);
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if (tag != BLK_MQ_NO_TAG)
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break;
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sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
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tag = __blk_mq_get_tag(data, bt);
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if (tag != BLK_MQ_NO_TAG)
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break;
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bt_prev = bt;
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io_schedule();
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sbitmap_finish_wait(bt, ws, &wait);
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data->ctx = blk_mq_get_ctx(data->q);
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data->hctx = blk_mq_map_queue(data->q, data->cmd_flags,
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data->ctx);
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tags = blk_mq_tags_from_data(data);
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if (data->flags & BLK_MQ_REQ_RESERVED)
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bt = tags->breserved_tags;
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else
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bt = tags->bitmap_tags;
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/*
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* If destination hw queue is changed, fake wake up on
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* previous queue for compensating the wake up miss, so
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* other allocations on previous queue won't be starved.
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*/
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if (bt != bt_prev)
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sbitmap_queue_wake_up(bt_prev);
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ws = bt_wait_ptr(bt, data->hctx);
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} while (1);
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sbitmap_finish_wait(bt, ws, &wait);
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found_tag:
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/*
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* Give up this allocation if the hctx is inactive. The caller will
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* retry on an active hctx.
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*/
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if (unlikely(test_bit(BLK_MQ_S_INACTIVE, &data->hctx->state))) {
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blk_mq_put_tag(tags, data->ctx, tag + tag_offset);
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return BLK_MQ_NO_TAG;
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}
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return tag + tag_offset;
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}
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void blk_mq_put_tag(struct blk_mq_tags *tags, struct blk_mq_ctx *ctx,
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unsigned int tag)
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{
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if (!blk_mq_tag_is_reserved(tags, tag)) {
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const int real_tag = tag - tags->nr_reserved_tags;
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BUG_ON(real_tag >= tags->nr_tags);
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sbitmap_queue_clear(tags->bitmap_tags, real_tag, ctx->cpu);
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} else {
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BUG_ON(tag >= tags->nr_reserved_tags);
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sbitmap_queue_clear(tags->breserved_tags, tag, ctx->cpu);
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}
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}
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struct bt_iter_data {
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struct blk_mq_hw_ctx *hctx;
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busy_iter_fn *fn;
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void *data;
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bool reserved;
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};
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static struct request *blk_mq_find_and_get_req(struct blk_mq_tags *tags,
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unsigned int bitnr)
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{
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struct request *rq;
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unsigned long flags;
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spin_lock_irqsave(&tags->lock, flags);
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rq = tags->rqs[bitnr];
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if (!rq || !refcount_inc_not_zero(&rq->ref))
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rq = NULL;
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spin_unlock_irqrestore(&tags->lock, flags);
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return rq;
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}
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static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
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{
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struct bt_iter_data *iter_data = data;
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struct blk_mq_hw_ctx *hctx = iter_data->hctx;
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struct blk_mq_tags *tags = hctx->tags;
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bool reserved = iter_data->reserved;
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struct request *rq;
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bool ret = true;
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if (!reserved)
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bitnr += tags->nr_reserved_tags;
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/*
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* We can hit rq == NULL here, because the tagging functions
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* test and set the bit before assigning ->rqs[].
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*/
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rq = blk_mq_find_and_get_req(tags, bitnr);
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if (!rq)
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return true;
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if (rq->q == hctx->queue && rq->mq_hctx == hctx)
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ret = iter_data->fn(hctx, rq, iter_data->data, reserved);
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blk_mq_put_rq_ref(rq);
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return ret;
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}
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/**
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* bt_for_each - iterate over the requests associated with a hardware queue
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* @hctx: Hardware queue to examine.
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* @bt: sbitmap to examine. This is either the breserved_tags member
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* or the bitmap_tags member of struct blk_mq_tags.
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* @fn: Pointer to the function that will be called for each request
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* associated with @hctx that has been assigned a driver tag.
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* @fn will be called as follows: @fn(@hctx, rq, @data, @reserved)
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* where rq is a pointer to a request. Return true to continue
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* iterating tags, false to stop.
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* @data: Will be passed as third argument to @fn.
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* @reserved: Indicates whether @bt is the breserved_tags member or the
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* bitmap_tags member of struct blk_mq_tags.
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*/
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static void bt_for_each(struct blk_mq_hw_ctx *hctx, struct sbitmap_queue *bt,
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busy_iter_fn *fn, void *data, bool reserved)
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{
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struct bt_iter_data iter_data = {
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.hctx = hctx,
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.fn = fn,
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.data = data,
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.reserved = reserved,
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};
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sbitmap_for_each_set(&bt->sb, bt_iter, &iter_data);
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}
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struct bt_tags_iter_data {
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struct blk_mq_tags *tags;
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busy_tag_iter_fn *fn;
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void *data;
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unsigned int flags;
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};
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#define BT_TAG_ITER_RESERVED (1 << 0)
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#define BT_TAG_ITER_STARTED (1 << 1)
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#define BT_TAG_ITER_STATIC_RQS (1 << 2)
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static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
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{
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struct bt_tags_iter_data *iter_data = data;
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struct blk_mq_tags *tags = iter_data->tags;
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bool reserved = iter_data->flags & BT_TAG_ITER_RESERVED;
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struct request *rq;
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bool ret = true;
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bool iter_static_rqs = !!(iter_data->flags & BT_TAG_ITER_STATIC_RQS);
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if (!reserved)
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bitnr += tags->nr_reserved_tags;
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/*
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* We can hit rq == NULL here, because the tagging functions
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* test and set the bit before assigning ->rqs[].
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*/
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if (iter_static_rqs)
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rq = tags->static_rqs[bitnr];
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else
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rq = blk_mq_find_and_get_req(tags, bitnr);
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if (!rq)
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return true;
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if (!(iter_data->flags & BT_TAG_ITER_STARTED) ||
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blk_mq_request_started(rq))
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ret = iter_data->fn(rq, iter_data->data, reserved);
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if (!iter_static_rqs)
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blk_mq_put_rq_ref(rq);
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return ret;
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}
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/**
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* bt_tags_for_each - iterate over the requests in a tag map
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* @tags: Tag map to iterate over.
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* @bt: sbitmap to examine. This is either the breserved_tags member
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* or the bitmap_tags member of struct blk_mq_tags.
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* @fn: Pointer to the function that will be called for each started
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* request. @fn will be called as follows: @fn(rq, @data,
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* @reserved) where rq is a pointer to a request. Return true
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* to continue iterating tags, false to stop.
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* @data: Will be passed as second argument to @fn.
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* @flags: BT_TAG_ITER_*
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*/
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static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt,
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busy_tag_iter_fn *fn, void *data, unsigned int flags)
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{
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struct bt_tags_iter_data iter_data = {
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.tags = tags,
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.fn = fn,
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.data = data,
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.flags = flags,
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};
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if (tags->rqs)
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sbitmap_for_each_set(&bt->sb, bt_tags_iter, &iter_data);
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}
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static void __blk_mq_all_tag_iter(struct blk_mq_tags *tags,
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busy_tag_iter_fn *fn, void *priv, unsigned int flags)
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{
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WARN_ON_ONCE(flags & BT_TAG_ITER_RESERVED);
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if (tags->nr_reserved_tags)
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bt_tags_for_each(tags, tags->breserved_tags, fn, priv,
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flags | BT_TAG_ITER_RESERVED);
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bt_tags_for_each(tags, tags->bitmap_tags, fn, priv, flags);
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}
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/**
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* blk_mq_all_tag_iter - iterate over all requests in a tag map
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* @tags: Tag map to iterate over.
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* @fn: Pointer to the function that will be called for each
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* request. @fn will be called as follows: @fn(rq, @priv,
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* reserved) where rq is a pointer to a request. 'reserved'
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* indicates whether or not @rq is a reserved request. Return
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* true to continue iterating tags, false to stop.
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* @priv: Will be passed as second argument to @fn.
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*
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* Caller has to pass the tag map from which requests are allocated.
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*/
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void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
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void *priv)
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{
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__blk_mq_all_tag_iter(tags, fn, priv, BT_TAG_ITER_STATIC_RQS);
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}
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/**
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* blk_mq_tagset_busy_iter - iterate over all started requests in a tag set
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* @tagset: Tag set to iterate over.
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* @fn: Pointer to the function that will be called for each started
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* request. @fn will be called as follows: @fn(rq, @priv,
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* reserved) where rq is a pointer to a request. 'reserved'
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* indicates whether or not @rq is a reserved request. Return
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* true to continue iterating tags, false to stop.
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* @priv: Will be passed as second argument to @fn.
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*
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* We grab one request reference before calling @fn and release it after
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* @fn returns.
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*/
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void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
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busy_tag_iter_fn *fn, void *priv)
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{
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int i;
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for (i = 0; i < tagset->nr_hw_queues; i++) {
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if (tagset->tags && tagset->tags[i])
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__blk_mq_all_tag_iter(tagset->tags[i], fn, priv,
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BT_TAG_ITER_STARTED);
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}
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}
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EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
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static bool blk_mq_tagset_count_completed_rqs(struct request *rq,
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void *data, bool reserved)
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{
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unsigned *count = data;
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if (blk_mq_request_completed(rq))
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(*count)++;
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return true;
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}
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/**
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* blk_mq_tagset_wait_completed_request - Wait until all scheduled request
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* completions have finished.
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* @tagset: Tag set to drain completed request
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*
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* Note: This function has to be run after all IO queues are shutdown
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*/
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void blk_mq_tagset_wait_completed_request(struct blk_mq_tag_set *tagset)
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{
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while (true) {
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unsigned count = 0;
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blk_mq_tagset_busy_iter(tagset,
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blk_mq_tagset_count_completed_rqs, &count);
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if (!count)
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break;
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msleep(5);
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}
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}
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EXPORT_SYMBOL(blk_mq_tagset_wait_completed_request);
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/**
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* blk_mq_queue_tag_busy_iter - iterate over all requests with a driver tag
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* @q: Request queue to examine.
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* @fn: Pointer to the function that will be called for each request
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* on @q. @fn will be called as follows: @fn(hctx, rq, @priv,
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* reserved) where rq is a pointer to a request and hctx points
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* to the hardware queue associated with the request. 'reserved'
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* indicates whether or not @rq is a reserved request.
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* @priv: Will be passed as third argument to @fn.
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*
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* Note: if @q->tag_set is shared with other request queues then @fn will be
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* called for all requests on all queues that share that tag set and not only
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* for requests associated with @q.
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*/
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void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
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void *priv)
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{
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struct blk_mq_hw_ctx *hctx;
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int i;
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/*
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* __blk_mq_update_nr_hw_queues() updates nr_hw_queues and queue_hw_ctx
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* while the queue is frozen. So we can use q_usage_counter to avoid
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* racing with it.
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*/
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if (!percpu_ref_tryget(&q->q_usage_counter))
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return;
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queue_for_each_hw_ctx(q, hctx, i) {
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struct blk_mq_tags *tags = hctx->tags;
|
|
|
|
/*
|
|
* If no software queues are currently mapped to this
|
|
* hardware queue, there's nothing to check
|
|
*/
|
|
if (!blk_mq_hw_queue_mapped(hctx))
|
|
continue;
|
|
|
|
if (tags->nr_reserved_tags)
|
|
bt_for_each(hctx, tags->breserved_tags, fn, priv, true);
|
|
bt_for_each(hctx, tags->bitmap_tags, fn, priv, false);
|
|
}
|
|
blk_queue_exit(q);
|
|
}
|
|
|
|
static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,
|
|
bool round_robin, int node)
|
|
{
|
|
return sbitmap_queue_init_node(bt, depth, -1, round_robin, GFP_KERNEL,
|
|
node);
|
|
}
|
|
|
|
int blk_mq_init_bitmaps(struct sbitmap_queue *bitmap_tags,
|
|
struct sbitmap_queue *breserved_tags,
|
|
unsigned int queue_depth, unsigned int reserved,
|
|
int node, int alloc_policy)
|
|
{
|
|
unsigned int depth = queue_depth - reserved;
|
|
bool round_robin = alloc_policy == BLK_TAG_ALLOC_RR;
|
|
|
|
if (bt_alloc(bitmap_tags, depth, round_robin, node))
|
|
return -ENOMEM;
|
|
if (bt_alloc(breserved_tags, reserved, round_robin, node))
|
|
goto free_bitmap_tags;
|
|
|
|
return 0;
|
|
|
|
free_bitmap_tags:
|
|
sbitmap_queue_free(bitmap_tags);
|
|
return -ENOMEM;
|
|
}
|
|
|
|
static int blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
|
|
int node, int alloc_policy)
|
|
{
|
|
int ret;
|
|
|
|
ret = blk_mq_init_bitmaps(&tags->__bitmap_tags,
|
|
&tags->__breserved_tags,
|
|
tags->nr_tags, tags->nr_reserved_tags,
|
|
node, alloc_policy);
|
|
if (ret)
|
|
return ret;
|
|
|
|
tags->bitmap_tags = &tags->__bitmap_tags;
|
|
tags->breserved_tags = &tags->__breserved_tags;
|
|
|
|
return 0;
|
|
}
|
|
|
|
int blk_mq_init_shared_sbitmap(struct blk_mq_tag_set *set)
|
|
{
|
|
int alloc_policy = BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags);
|
|
int i, ret;
|
|
|
|
ret = blk_mq_init_bitmaps(&set->__bitmap_tags, &set->__breserved_tags,
|
|
set->queue_depth, set->reserved_tags,
|
|
set->numa_node, alloc_policy);
|
|
if (ret)
|
|
return ret;
|
|
|
|
for (i = 0; i < set->nr_hw_queues; i++) {
|
|
struct blk_mq_tags *tags = set->tags[i];
|
|
|
|
tags->bitmap_tags = &set->__bitmap_tags;
|
|
tags->breserved_tags = &set->__breserved_tags;
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
|
|
void blk_mq_exit_shared_sbitmap(struct blk_mq_tag_set *set)
|
|
{
|
|
sbitmap_queue_free(&set->__bitmap_tags);
|
|
sbitmap_queue_free(&set->__breserved_tags);
|
|
}
|
|
|
|
struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
|
|
unsigned int reserved_tags,
|
|
int node, unsigned int flags)
|
|
{
|
|
int alloc_policy = BLK_MQ_FLAG_TO_ALLOC_POLICY(flags);
|
|
struct blk_mq_tags *tags;
|
|
|
|
if (total_tags > BLK_MQ_TAG_MAX) {
|
|
pr_err("blk-mq: tag depth too large\n");
|
|
return NULL;
|
|
}
|
|
|
|
tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
|
|
if (!tags)
|
|
return NULL;
|
|
|
|
tags->nr_tags = total_tags;
|
|
tags->nr_reserved_tags = reserved_tags;
|
|
spin_lock_init(&tags->lock);
|
|
|
|
if (blk_mq_is_sbitmap_shared(flags))
|
|
return tags;
|
|
|
|
if (blk_mq_init_bitmap_tags(tags, node, alloc_policy) < 0) {
|
|
kfree(tags);
|
|
return NULL;
|
|
}
|
|
return tags;
|
|
}
|
|
|
|
void blk_mq_free_tags(struct blk_mq_tags *tags, unsigned int flags)
|
|
{
|
|
if (!blk_mq_is_sbitmap_shared(flags)) {
|
|
sbitmap_queue_free(tags->bitmap_tags);
|
|
sbitmap_queue_free(tags->breserved_tags);
|
|
}
|
|
kfree(tags);
|
|
}
|
|
|
|
int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
|
|
struct blk_mq_tags **tagsptr, unsigned int tdepth,
|
|
bool can_grow)
|
|
{
|
|
struct blk_mq_tags *tags = *tagsptr;
|
|
|
|
if (tdepth <= tags->nr_reserved_tags)
|
|
return -EINVAL;
|
|
|
|
/*
|
|
* If we are allowed to grow beyond the original size, allocate
|
|
* a new set of tags before freeing the old one.
|
|
*/
|
|
if (tdepth > tags->nr_tags) {
|
|
struct blk_mq_tag_set *set = hctx->queue->tag_set;
|
|
struct blk_mq_tags *new;
|
|
bool ret;
|
|
|
|
if (!can_grow)
|
|
return -EINVAL;
|
|
|
|
/*
|
|
* We need some sort of upper limit, set it high enough that
|
|
* no valid use cases should require more.
|
|
*/
|
|
if (tdepth > MAX_SCHED_RQ)
|
|
return -EINVAL;
|
|
|
|
new = blk_mq_alloc_rq_map(set, hctx->queue_num, tdepth,
|
|
tags->nr_reserved_tags, set->flags);
|
|
if (!new)
|
|
return -ENOMEM;
|
|
ret = blk_mq_alloc_rqs(set, new, hctx->queue_num, tdepth);
|
|
if (ret) {
|
|
blk_mq_free_rq_map(new, set->flags);
|
|
return -ENOMEM;
|
|
}
|
|
|
|
blk_mq_free_rqs(set, *tagsptr, hctx->queue_num);
|
|
blk_mq_free_rq_map(*tagsptr, set->flags);
|
|
*tagsptr = new;
|
|
} else {
|
|
/*
|
|
* Don't need (or can't) update reserved tags here, they
|
|
* remain static and should never need resizing.
|
|
*/
|
|
sbitmap_queue_resize(tags->bitmap_tags,
|
|
tdepth - tags->nr_reserved_tags);
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
|
|
void blk_mq_tag_resize_shared_sbitmap(struct blk_mq_tag_set *set, unsigned int size)
|
|
{
|
|
sbitmap_queue_resize(&set->__bitmap_tags, size - set->reserved_tags);
|
|
}
|
|
|
|
/**
|
|
* blk_mq_unique_tag() - return a tag that is unique queue-wide
|
|
* @rq: request for which to compute a unique tag
|
|
*
|
|
* The tag field in struct request is unique per hardware queue but not over
|
|
* all hardware queues. Hence this function that returns a tag with the
|
|
* hardware context index in the upper bits and the per hardware queue tag in
|
|
* the lower bits.
|
|
*
|
|
* Note: When called for a request that is queued on a non-multiqueue request
|
|
* queue, the hardware context index is set to zero.
|
|
*/
|
|
u32 blk_mq_unique_tag(struct request *rq)
|
|
{
|
|
return (rq->mq_hctx->queue_num << BLK_MQ_UNIQUE_TAG_BITS) |
|
|
(rq->tag & BLK_MQ_UNIQUE_TAG_MASK);
|
|
}
|
|
EXPORT_SYMBOL(blk_mq_unique_tag);
|