linux-kernel - Re: [PATCH -next v2 2/4] blk-throttle: fix wrong comparation while 'carryover

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Message-ID: <ZO5uxJSlHRxexZwk@slm.duckdns.org>
Date:   Tue, 29 Aug 2023 12:18:44 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     josef@...icpanda.com, axboe@...nel.dk, yukuai3@...wei.com,
        mkoutny@...e.com, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next v2 2/4] blk-throttle: fix wrong comparation while
 'carryover_ios/bytes' is negative

On Wed, Aug 16, 2023 at 09:27:06AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> carryover_ios/bytes[] can be negative in the case that ios are
> dispatched in the slice in advance, and then configuration is updated.
> For example:
> 
> 1) set iops limit to 1000, and slice start is 0, slice end is 100ms;
> 2) current time is 0, and 100 ios are dispatched, those ios will not be
>    throttled, hence io_disp is 100;
> 3) still at current time 0, update iops limit to 100, then carryover_ios
>    is (0 - 100) = -100;
> 4) then, dispatch a new io at time 0, the expected result is that this
>    io will wait for 1s. The calculation in tg_within_iops_limit:
> 
>    io_disp = 0;
>    io_allowed = calculate_io_allowed + carryover_ios
> 	      = 10 + (-100) = -90;
>    io won't be throttled if (io_disp + 1 < io_allowed) passed.
> 
> Before this patch, in step 4) (io_disp + 1 < io_allowed) is passed,
> because -90 for unsigned value is very huge, and such io won't be
> throttled.
> 
> Fix this problem by checking if 'io/bytes_allowed' is negative first.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

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

Thanks.

-- 
tejun