linux-kernel - [PATCH RFC -next 0/3] improve fairness for sbitmap waitqueues

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Date:   Fri, 18 Mar 2022 16:25:02 +0800
From:   Yu Kuai <yukuai3@...wei.com>
To:     <axboe@...nel.dk>, <ming.lei@...hat.com>,
        <andriy.shevchenko@...ux.intel.com>, <john.garry@...wei.com>,
        <yukuai3@...wei.com>, <bvanassche@....org>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: [PATCH RFC -next 0/3] improve fairness for sbitmap waitqueues

During some io test, I found that waitqueues can be extremly unbalanced,
especially when tags are little.

For example:
test cmd: nr_requests is set to 64, and queue_depth is set to 32
[global]
filename=/dev/sdh
ioengine=libaio
direct=1
allow_mounted_write=0
group_reporting

[test]
rw=randwrite
bs=4k
numjobs=512
iodepth=2

With patch 1 applied, I observe the following status:
ws_active=484
ws={
        {.wait_cnt=8, .waiters_cnt=117},
        {.wait_cnt=8, .waiters_cnt=59},
        {.wait_cnt=8, .waiters_cnt=76},
        {.wait_cnt=8, .waiters_cnt=0},
        {.wait_cnt=5, .waiters_cnt=24},
        {.wait_cnt=8, .waiters_cnt=12},
        {.wait_cnt=8, .waiters_cnt=21},
        {.wait_cnt=8, .waiters_cnt=175},
}

'waiters_cnt' means how many threads are waitng for tags in the 'ws',
and such extremely unbalanced status is very frequent. After reading the
sbitmap code, I found there are two situations that might cause the
problem:

1) blk_mq_get_tag() can call 'bt_wait_ptr()' while the threads might get
tag successfully before going to wait. - patch 2

2) After a 'ws' is woken up, following blk_mq_put_tag() might wake up
the same 'ws' again instead of the next one. - patch 3

I'm not sure if the unbalanced status is really a *problem* and need to
be fixed, this patchset is just to improve fairness and not a thorough
fix. Any comments and suggestions are welcome.

Yu Kuai (3):
  sbitmap: record the number of waiters for each waitqueue
  blk-mq: call 'bt_wait_ptr()' later in blk_mq_get_tag()
  sbitmap: improve the fairness of waitqueues' wake up

 block/blk-mq-tag.c      |  6 ++---
 include/linux/sbitmap.h |  5 ++++
 lib/sbitmap.c           | 57 ++++++++++++++++++++++-------------------
 3 files changed, 39 insertions(+), 29 deletions(-)

-- 
2.31.1