linux-kernel - Re: [PATCH v2] blk-throttle: fix lower control under super low iops limit

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Message-ID: <19568a6b-66a8-bb93-7c8c-3b523972535a@huaweicloud.com>
Date: Fri, 28 Jun 2024 11:34:32 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Yu Kuai <yukuai1@...weicloud.com>, tj@...nel.org, josef@...icpanda.com,
 axboe@...nel.dk
Cc: cgroups@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2] blk-throttle: fix lower control under super low iops
 limit

Hi,

在 2024/06/18 14:21, Yu Kuai 写道:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> User will configure allowed iops limit in 1s, and calculate_io_allowed()
> will calculate allowed iops in the slice by:
> 
> limit * HZ / throtl_slice
> 
> However, if limit is quite low, the result can be 0, then
> allowed IO in the slice is 0, this will cause missing dispatch and
> control will be lower than limit.
> 
> For example, set iops_limit to 5 with HD disk, and test will found that
> iops will be 3.
> 
> This is usually not a big deal, because user will unlikely to configure
> such low iops limit, however, this is still a problem in the extreme
> scene.
> 
> Fix the problem by making sure the wait time calculated by
> tg_within_iops_limit() should allow at least one IO to be dispatched.

Friendly ping ...
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
> Changes in v2:
>   - instead of extend thorlt_slice, extend wait time;
>   block/blk-throttle.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index c1bf73f8c75d..dc6140fa3de0 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -704,6 +704,9 @@ static unsigned long tg_within_iops_limit(struct throtl_grp *tg, struct bio *bio
>   
>   	/* Calc approx time to dispatch */
>   	jiffy_wait = jiffy_elapsed_rnd - jiffy_elapsed;
> +
> +	/* make sure at least one io can be dispatched after waiting */
> +	jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1);
>   	return jiffy_wait;
>   }
>   
>