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

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Date: Fri, 28 Jun 2024 07:16:51 -1000
From: Tejun Heo <tj@...nel.org>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: josef@...icpanda.com, axboe@...nel.dk, cgroups@...r.kernel.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v2] blk-throttle: fix lower control under super low iops
 limit

Sorry about the delay.

On Tue, Jun 18, 2024 at 02:21:08PM +0800, Yu Kuai wrote:
> 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.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun