linux-kernel - Re: [PATCH RFC 5/7] blk-mq: record the number of times fail to get driver tag while sharing tags

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Date:   Thu, 6 Jul 2023 11:18:41 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH RFC 5/7] blk-mq: record the number of times fail to get
 driver tag while sharing tags

On 6/18/23 09:07, Yu Kuai wrote:
> +static void update_tag_sharing_busy(struct tag_sharing *tag_sharing)
> +{
> +	unsigned int count = atomic_inc_return(&tag_sharing->fail_count);
> +	unsigned long last_period = READ_ONCE(tag_sharing->period);
> +
> +	if (time_after(jiffies, last_period + HZ) &&
> +	    cmpxchg_relaxed(&tag_sharing->period, last_period, jiffies) ==
> +			    last_period)
> +		atomic_sub(count / 2, &tag_sharing->fail_count);
> +}

For new code, try_cmpxchg_relaxed() is preferred over cmpxchg_relaxed().

>   struct tag_sharing {
>   	struct list_head	node;
>   	unsigned int		available_tags;
> +	atomic_t		fail_count;
> +	unsigned long		period;
>   };

Please consider renaming "period" into "latest_reduction" or any other name
that make the purpose of this member clear.

Thanks,

Bart.