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

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Message-ID: <c4d4b335-90b9-0211-acb4-b80f3bb5de5e@huawei.com>
Date:   Fri, 1 Apr 2022 11:43:22 +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 2022/03/25 15:30, yukuai (C) 写道:
> 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(-)
>>