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

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Date:   Fri, 25 Mar 2022 15:30:02 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     <axboe@...nel.dk>, <ming.lei@...hat.com>,
        <andriy.shevchenko@...ux.intel.com>, <john.garry@...wei.com>,
        <bvanassche@....org>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: Re: [PATCH RFC -next 0/3] improve fairness for sbitmap waitqueues

friendly ping ...

在 2022/03/18 16:25, Yu Kuai 写道:
> 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(-)
>