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

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Date:   Fri, 8 Apr 2022 07:24:47 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Yu Kuai <yukuai3@...wei.com>, 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

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,

Bart.