linux-kernel - Re: [PATCH RESEND v6 1/8] blk-throttle: fix that io throttle can only work for single bio

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Date:   Sat, 30 Jul 2022 09:06:37 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Tejun Heo <tj@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     mkoutny@...e.com, axboe@...nel.dk, ming.lei@...hat.com,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RESEND v6 1/8] blk-throttle: fix that io throttle can only
 work for single bio



在 2022/07/30 2:04, Tejun Heo 写道:
> Hello,
> 
> On Fri, Jul 29, 2022 at 02:32:36PM +0800, Yu Kuai wrote:
>> We need to make sure following conditions is always hold:
>>
>> 1) If a bio is splited, iops limits should count multiple times, while
>> bps limits should only count once.
>> 2) If a bio is issued while some bios are already throttled, bps limits
>> should not be ignored.
>>
>> commit 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
>> fixes that 1) is not hold, while it breaks 2). Root cause is that such
>> bio will be flaged in __blk_throtl_bio(), and later
>> tg_with_in_bps_limit() will skip flaged bio.
>>
>> In order to fix this problem, at first, I change that flaged bio won't
>> be skipped in tg_with_in_bps_limit():
>>
>> -	if (!bio_flagged(bio, BIO_THROTTLED)) {
>> -		tg->bytes_disp[rw] += bio_size;
>> -		tg->last_bytes_disp[rw] += bio_size;
>> -	}
>> -
>> +	tg->bytes_disp[rw] += bio_size;
>> +	tg->last_bytes_disp[rw] += bio_size;
>>
>> However, this will break that bps limits should only count once. Thus I
>> try to restore the overaccounting in __blk_throtl_bio() in such case:
>>
>> +		if (bio_flagged(bio, BIO_THROTTLED)) {
>> +			unsigned int bio_size = throtl_bio_data_size(bio);
>> +
>> +			if (tg->bytes_disp[rw] >= bio_size)
>> +				tg->bytes_disp[rw] -= bio_size;
>> +			if (tg->last_bytes_disp[rw] >= bio_size)
>> +				tg->last_bytes_disp[rw] -= bio_size;
>> +		}
>>
>> If new slice is not started, then the decrement should make sure this
>> bio won't be counted again. However, if new slice is started and the
>> condition 'bytes_disp >= bio_size' doesn't hold, this bio will end up
>> accounting twice.
>>
>> Pleas let me know if you think this suituation is problematic, I'll try
>> to figure out a new way...
> 
> While a bit tricky, I think it's fine but please add comments in the code
> explaining what's going on and why. Also, can you please explain why
> __blk_throtl_bio() being skipped when iops limit is not set doesn't skew the
> result?

Because bps limit is already counted the first time __blk_throtl_bio()
is called for the orignal bio. When splited bio is reentered, we only
need to throttle it again if iops limit is set.

By the way, I found that this way is better after patch 4:

in __blk_throtl_bio():

if (bio_flagged(bio, BIO_THROTTLED)) {
	tg->bytes_skipped[rw] += bio_size;
	if (tg->last_bytes_disp[rw] >= bio_size)
		tg->last_bytes_disp[rw] -= bio_size;
}

The overaccounting can be restored even if new slice is started.

Thanks,
Kuai