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blk-mq: Use request queue-wide tags for tagset-wide sbitmap

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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>
This commit is contained in:
John Garry
2021-05-13 20:00:58 +08:00
committed by Jens Axboe
parent 56b68085e5
commit d97e594c51
5 changed files with 76 additions and 21 deletions
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4
include/linux/blkdev.h
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@@ -25,6 +25,7 @@
#include <linux/scatterlist.h>
#include <linux/blkzoned.h>
#include <linux/pm.h>
#include <linux/sbitmap.h>
struct module;
struct scsi_ioctl_command;
@@ -493,6 +494,9 @@ struct request_queue {
atomic_t nr_active_requests_shared_sbitmap;
struct sbitmap_queue sched_bitmap_tags;
struct sbitmap_queue sched_breserved_tags;
struct list_head icq_list;
#ifdef CONFIG_BLK_CGROUP
DECLARE_BITMAP (blkcg_pols, BLKCG_MAX_POLS);