linux-kernel - Re: [PATCH -next RFC v2 4/8] blk-mq: don't preempt tag under heavy load

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Date:   Sat, 9 Apr 2022 10:38:14 +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 4/8] blk-mq: don't preempt tag under heavy
 load

在 2022/04/08 22:24, Bart Van Assche 写道:
> On 4/8/22 00:39, Yu Kuai wrote:
>> The idle way to disable tag preemption is to track how many tags are
> 
> idle -> ideal?
> 
>> available, and wait directly in blk_mq_get_tag() if free tags are
>> very little. However, this is out of reality because fast path is
>> affected.
>>
>> As 'ws_active' is only updated in slow path, this patch disable tag
>> preemption if 'ws_active' is greater than 8, which means there are many
>> threads waiting for tags already.
>>
>> Once tag preemption is disabled, there is a situation that can cause
>> performance degration(or io hung in extreme scenarios): the waitqueue
> 
> degration -> degradation?
> 
>> diff --git a/block/blk-mq.h b/block/blk-mq.h
>> index 2615bd58bad3..b49b20e11350 100644
>> --- a/block/blk-mq.h
>> +++ b/block/blk-mq.h
>> @@ -156,6 +156,7 @@ struct blk_mq_alloc_data {
>>       /* allocate multiple requests/tags in one go */
>>       unsigned int nr_tags;
>> +    bool preemption;
>>       struct request **cached_rq;
> 
> Please change "preemption" into "preempt".

Thanks for your advice, will change that and previous spelling mistakes.

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