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

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(-)
> 

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(-)
>>