linux-kernel - Re: [PATCH] blk-iocost: fix very large vtime when iocg activate

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Message-ID: <YoKb7wpkz3xoCS6s@slm.duckdns.org>
Date:   Mon, 16 May 2022 08:46:07 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, duanxiongchun@...edance.com,
        songmuchun@...edance.com
Subject: Re: [PATCH] blk-iocost: fix very large vtime when iocg activate

On Mon, May 16, 2022 at 04:40:45PM +0800, Chengming Zhou wrote:
> When the first iocg activate after blk_iocost_init(), now->vnow
> maybe smaller than ioc->margins.target, cause very large vtarget
> since it's u64.
> 
> 	vtarget = now->vnow - ioc->margins.target;
> 	atomic64_add(vtarget - vtime, &iocg->vtime);
> 
> Then the iocg's vtime will be very large too, larger than now->vnow.

It's a wrapping counter. Please take a look at how time_before64() and
friends work.

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

Again, please spend more effort understanding the code before sending these
subtle patches.

Thanks.

-- 
tejun