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![Jens Axboe](/assets/img/avatar_default.png)
Even with a number of waitqueues, we can get into a situation where we are heavily contended on the waitqueue lock. I got a report on spc1 where we're spending seconds doing this. Arguably the use case is nasty, I reproduce it with one device and 1000 threads banging on the device. But that doesn't mean we shouldn't be handling it better. What ends up happening is that a thread will fail to get a tag, add itself to the waitqueue, and subsequently get woken up when a tag is freed - only to find itself going back to sleep on the waitqueue. Instead of waking all threads, use an exclusive wait and wake up our sbitmap batch count instead. This seems to work well for me (massive improvement for this use case), and it survives basic testing. But I haven't fully verified it yet. An additional improvement is running the queue and checking for a new tag BEFORE needing to add ourselves to the waitqueue. Signed-off-by: Jens Axboe <axboe@kernel.dk>
489 lines
11 KiB
C
489 lines
11 KiB
C
/*
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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 "blk.h"
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#include "blk-mq.h"
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#include "blk-mq-tag.h"
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bool blk_mq_has_free_tags(struct blk_mq_tags *tags)
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{
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if (!tags)
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return true;
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return sbitmap_any_bit_clear(&tags->bitmap_tags.sb);
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}
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/*
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* If a previously inactive queue goes active, bump the active user count.
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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 (!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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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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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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blk_mq_tag_wakeup_all(tags, false);
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}
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/*
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* For shared tag users, we track the number of currently active users
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* and attempt to provide a fair share of the tag depth for each of them.
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*/
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static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
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struct sbitmap_queue *bt)
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{
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unsigned int depth, users;
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if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_SHARED))
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return true;
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if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
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return true;
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/*
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* Don't try dividing an ant
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*/
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if (bt->sb.depth == 1)
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return true;
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users = atomic_read(&hctx->tags->active_queues);
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if (!users)
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return true;
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/*
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* Allow at least some tags
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*/
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depth = max((bt->sb.depth + users - 1) / users, 4U);
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return atomic_read(&hctx->nr_active) < depth;
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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->flags & BLK_MQ_REQ_INTERNAL) &&
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!hctx_may_queue(data->hctx, bt))
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return -1;
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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_WAIT(wait);
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unsigned int tag_offset;
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bool drop_ctx;
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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_TAG_FAIL;
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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 != -1)
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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_TAG_FAIL;
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ws = bt_wait_ptr(bt, data->hctx);
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drop_ctx = data->ctx == NULL;
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do {
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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 != -1)
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break;
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prepare_to_wait_exclusive(&ws->wait, &wait,
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TASK_UNINTERRUPTIBLE);
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tag = __blk_mq_get_tag(data, bt);
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if (tag != -1)
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break;
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if (data->ctx)
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blk_mq_put_ctx(data->ctx);
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io_schedule();
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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->ctx->cpu);
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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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finish_wait(&ws->wait, &wait);
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ws = bt_wait_ptr(bt, data->hctx);
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} while (1);
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if (drop_ctx && data->ctx)
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blk_mq_put_ctx(data->ctx);
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finish_wait(&ws->wait, &wait);
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found_tag:
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return tag + tag_offset;
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}
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void blk_mq_put_tag(struct blk_mq_hw_ctx *hctx, struct blk_mq_tags *tags,
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struct blk_mq_ctx *ctx, 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 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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if (!reserved)
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bitnr += tags->nr_reserved_tags;
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rq = tags->rqs[bitnr];
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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 assining ->rqs[].
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*/
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if (rq && rq->q == hctx->queue)
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iter_data->fn(hctx, rq, iter_data->data, reserved);
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return true;
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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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bool reserved;
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};
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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->reserved;
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struct request *rq;
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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 assining ->rqs[].
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*/
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rq = tags->rqs[bitnr];
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if (rq)
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iter_data->fn(rq, iter_data->data, reserved);
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return true;
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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, bool reserved)
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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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.reserved = reserved,
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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_busy_iter(struct blk_mq_tags *tags,
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busy_tag_iter_fn *fn, void *priv)
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{
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if (tags->nr_reserved_tags)
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bt_tags_for_each(tags, &tags->breserved_tags, fn, priv, true);
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bt_tags_for_each(tags, &tags->bitmap_tags, fn, priv, false);
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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_busy_iter(tagset->tags[i], fn, priv);
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}
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}
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EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
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int blk_mq_tagset_iter(struct blk_mq_tag_set *set, void *data,
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int (fn)(void *, struct request *))
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{
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int i, j, ret = 0;
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if (WARN_ON_ONCE(!fn))
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goto out;
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for (i = 0; i < set->nr_hw_queues; i++) {
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struct blk_mq_tags *tags = set->tags[i];
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if (!tags)
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continue;
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for (j = 0; j < tags->nr_tags; j++) {
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if (!tags->static_rqs[j])
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continue;
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ret = fn(data, tags->static_rqs[j]);
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if (ret)
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goto out;
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}
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}
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out:
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return ret;
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}
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EXPORT_SYMBOL_GPL(blk_mq_tagset_iter);
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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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queue_for_each_hw_ctx(q, hctx, i) {
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struct blk_mq_tags *tags = hctx->tags;
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/*
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* If not software queues are currently mapped to this
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* hardware queue, there's nothing to check
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*/
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if (!blk_mq_hw_queue_mapped(hctx))
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continue;
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if (tags->nr_reserved_tags)
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bt_for_each(hctx, &tags->breserved_tags, fn, priv, true);
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bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false);
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}
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}
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static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,
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bool round_robin, int node)
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{
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return sbitmap_queue_init_node(bt, depth, -1, round_robin, GFP_KERNEL,
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node);
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}
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static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
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int node, int alloc_policy)
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{
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unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
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bool round_robin = alloc_policy == BLK_TAG_ALLOC_RR;
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if (bt_alloc(&tags->bitmap_tags, depth, round_robin, node))
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goto free_tags;
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if (bt_alloc(&tags->breserved_tags, tags->nr_reserved_tags, round_robin,
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node))
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goto free_bitmap_tags;
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return tags;
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free_bitmap_tags:
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sbitmap_queue_free(&tags->bitmap_tags);
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free_tags:
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kfree(tags);
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return NULL;
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}
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struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
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unsigned int reserved_tags,
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int node, int alloc_policy)
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{
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struct blk_mq_tags *tags;
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if (total_tags > BLK_MQ_TAG_MAX) {
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pr_err("blk-mq: tag depth too large\n");
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return NULL;
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}
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tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
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if (!tags)
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return NULL;
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tags->nr_tags = total_tags;
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tags->nr_reserved_tags = reserved_tags;
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return blk_mq_init_bitmap_tags(tags, node, alloc_policy);
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}
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void blk_mq_free_tags(struct blk_mq_tags *tags)
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{
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sbitmap_queue_free(&tags->bitmap_tags);
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sbitmap_queue_free(&tags->breserved_tags);
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kfree(tags);
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}
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int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
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struct blk_mq_tags **tagsptr, unsigned int tdepth,
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bool can_grow)
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{
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struct blk_mq_tags *tags = *tagsptr;
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if (tdepth <= tags->nr_reserved_tags)
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return -EINVAL;
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tdepth -= tags->nr_reserved_tags;
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/*
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* If we are allowed to grow beyond the original size, allocate
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* a new set of tags before freeing the old one.
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*/
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if (tdepth > tags->nr_tags) {
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struct blk_mq_tag_set *set = hctx->queue->tag_set;
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struct blk_mq_tags *new;
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bool ret;
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if (!can_grow)
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return -EINVAL;
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/*
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* We need some sort of upper limit, set it high enough that
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* no valid use cases should require more.
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*/
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if (tdepth > 16 * BLKDEV_MAX_RQ)
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return -EINVAL;
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new = blk_mq_alloc_rq_map(set, hctx->queue_num, tdepth, 0);
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if (!new)
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return -ENOMEM;
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ret = blk_mq_alloc_rqs(set, new, hctx->queue_num, tdepth);
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if (ret) {
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blk_mq_free_rq_map(new);
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return -ENOMEM;
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}
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blk_mq_free_rqs(set, *tagsptr, hctx->queue_num);
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blk_mq_free_rq_map(*tagsptr);
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*tagsptr = new;
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} else {
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/*
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* Don't need (or can't) update reserved tags here, they
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* remain static and should never need resizing.
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*/
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sbitmap_queue_resize(&tags->bitmap_tags, tdepth);
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}
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return 0;
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}
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/**
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* blk_mq_unique_tag() - return a tag that is unique queue-wide
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* @rq: request for which to compute a unique tag
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*
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* The tag field in struct request is unique per hardware queue but not over
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* all hardware queues. Hence this function that returns a tag with the
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* hardware context index in the upper bits and the per hardware queue tag in
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* the lower bits.
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*
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* Note: When called for a request that is queued on a non-multiqueue request
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* queue, the hardware context index is set to zero.
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*/
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u32 blk_mq_unique_tag(struct request *rq)
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{
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struct request_queue *q = rq->q;
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struct blk_mq_hw_ctx *hctx;
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int hwq = 0;
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if (q->mq_ops) {
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hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
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hwq = hctx->queue_num;
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}
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return (hwq << BLK_MQ_UNIQUE_TAG_BITS) |
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(rq->tag & BLK_MQ_UNIQUE_TAG_MASK);
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}
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EXPORT_SYMBOL(blk_mq_unique_tag);
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