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Subject: Re: [PATCH -next 2/2] blk-throttle: fix lower control under super low
 iops limit
To: Tejun Heo <tj@kernel.org>, Yu Kuai <yukuai1@huaweicloud.com>
Cc: josef@toxicpanda.com, axboe@kernel.dk, cgroups@vger.kernel.org,
 linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
 yi.zhang@huawei.com, yangerkun@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20240513120848.2828797-1-yukuai1@huaweicloud.com>
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From: Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <ff91d94e-8dcb-9722-d470-67e3eef1d219@huaweicloud.com>
Date: Fri, 14 Jun 2024 09:09:18 +0800
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Hi,

?? 2024/06/14 6:32, Tejun Heo д??:
> Hello,
> 
> On Mon, May 13, 2024 at 08:08:48PM +0800, Yu Kuai wrote:
> ...
>> 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.
> 
> Hmm... can't this be solved by starting the next slice with the right
> credit?

Of course, this looks like feasible. I'll look into this.
> 
>>   static unsigned int tg_throtl_slice(struct throtl_grp *tg, int rw)
>>   {
>> +	if (tg->throtl_slice[rw])
>> +		return tg->throtl_slice[rw];
>>   	return tg->td->throtl_slice;
> 
> Because this is a bit nasty. If we want to use difference throttling slices
> for different cgroups, we might as well do it universally.

I do thought about this, however, because different cgroups have
different slice start, I convinced myself this is fine to have different
slice as well.

Thanks,
Kuai

> 
> Thanks.
>