linux-kernel - Re: [PATCH -next RFC v2 2/8] blk-mq: call 'bt_wait_ptr()' later in blk_mq_get

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Message-ID: <fbe5565e-4dbe-bb71-e3c4-c33eb470a8d2@huawei.com>
Date:   Sat, 9 Apr 2022 10:09:52 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Bart Van Assche <bvanassche@....org>, <axboe@...nel.dk>,
        <andriy.shevchenko@...ux.intel.com>, <john.garry@...wei.com>,
        <ming.lei@...hat.com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: Re: [PATCH -next RFC v2 2/8] blk-mq: call 'bt_wait_ptr()' later in
 blk_mq_get_tag()

在 2022/04/08 22:20, Bart Van Assche 写道:
> On 4/8/22 00:39, Yu Kuai wrote:
>> bt_wait_ptr() will increase 'wait_index', however, if blk_mq_get_tag()
>> get a tag successfully after bt_wait_ptr() is called and before
>> sbitmap_prepare_to_wait() is called, then the 'ws' is skipped. This
>> behavior might cause 8 waitqueues to be unbalanced.
>>
>> Move bt_wait_ptr() later should reduce the problem when the disk is
>> under high io preesure.
>>
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> ---
>>   block/blk-mq-tag.c | 4 +---
>>   1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
>> index 68ac23d0b640..228a0001694f 100644
>> --- a/block/blk-mq-tag.c
>> +++ b/block/blk-mq-tag.c
>> @@ -155,7 +155,6 @@ unsigned int blk_mq_get_tag(struct 
>> blk_mq_alloc_data *data)
>>       if (data->flags & BLK_MQ_REQ_NOWAIT)
>>           return BLK_MQ_NO_TAG;
>> -    ws = bt_wait_ptr(bt, data->hctx);
>>       do {
>>           struct sbitmap_queue *bt_prev;
>> @@ -174,6 +173,7 @@ unsigned int blk_mq_get_tag(struct 
>> blk_mq_alloc_data *data)
>>           if (tag != BLK_MQ_NO_TAG)
>>               break;
>> +        ws = bt_wait_ptr(bt, data->hctx);
>>           sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
>>           tag = __blk_mq_get_tag(data, bt);
>> @@ -201,8 +201,6 @@ unsigned int blk_mq_get_tag(struct 
>> blk_mq_alloc_data *data)
>>            */
>>           if (bt != bt_prev)
>>               sbitmap_queue_wake_up(bt_prev);
>> -
>> -        ws = bt_wait_ptr(bt, data->hctx);
>>       } while (1);
> 
> Is it necessary to call bt_wait_ptr() during every loop iteration or 
> only if bt != bt_prev? Would calling bt_wait_ptr() only if bt != bt_prev 
> help to reduce unfairness further?
Hi,

You are right, that sounds reasonable.

Thanks,
Kuai
> 
> Thanks,
> 
> Bart.
> .
>