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

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Message-ID: <YuGDjFkxDSsVrcRw@slm.duckdns.org>
Date:   Wed, 27 Jul 2022 08:27:24 -1000
From:   Tejun Heo <tj@...nel.org>
To:     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, yukuai3@...wei.com,
        yi.zhang@...wei.com
Subject: Re: [PATCH RESEND v6 1/8] blk-throttle: fix that io throttle can
 only work for single bio

Sorry about the long delay.

So, the code looks nice but I have a difficult time following the logic.

On Fri, Jul 01, 2022 at 05:34:34PM +0800, Yu Kuai wrote:
> @@ -811,7 +811,7 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
>  	unsigned int bio_size = throtl_bio_data_size(bio);
>  
>  	/* no need to throttle if this bio's bytes have been accounted */
> -	if (bps_limit == U64_MAX || bio_flagged(bio, BIO_THROTTLED)) {
> +	if (bps_limit == U64_MAX) {
>  		if (wait)
>  			*wait = 0;
>  		return true;
> @@ -921,11 +921,8 @@ static void throtl_charge_bio(struct throtl_grp *tg, struct bio *bio)
>  	unsigned int bio_size = throtl_bio_data_size(bio);
>  
>  	/* Charge the bio to the group */
> -	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;
>  	tg->io_disp[rw]++;
>  	tg->last_io_disp[rw]++;

So, we're charging and controlling whether it has already been throttled or
not.

> @@ -2121,6 +2118,21 @@ bool __blk_throtl_bio(struct bio *bio)
>  			tg->last_low_overflow_time[rw] = jiffies;
>  		throtl_downgrade_check(tg);
>  		throtl_upgrade_check(tg);
> +
> +		/*
> +		 * re-entered bio has accounted bytes already, so try to
> +		 * compensate previous over-accounting. However, if new
> +		 * slice is started, just forget it.
> +		 */
> +		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;
> +		}

and trying to restore the overaccounting. However, it's not clear why this
helps with the problem you're describing. The comment should be clearly
spelling out why it's done this way and how this works.

Also, blk_throttl_bio() doesn't call into __blk_throtl_bio() at all if
THROTTLED is set and HAS_IOPS_LIMIT is not, so if there are only bw limits,
we end up accounting these IOs twice?

Thanks.

-- 
tejun