linux-kernel - Re: [PATCH] blk-iocost: fix false positive lagging

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Date:   Thu, 26 May 2022 07:43:07 -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
Subject: Re: [PATCH] blk-iocost: fix false positive lagging

Hello,

On Thu, May 26, 2022 at 09:35:54PM +0800, Chengming Zhou wrote:
> I found many false positive lagging during iocost test.
> 
> Since iocg->vtime will be advanced to (vnow - margins.target)
> in hweight_after_donation(), which called throw away excess,
> the iocg->done_vtime will also be advanced that much.
> 
>        period_at_vtime  <--period_vtime-->  vnow
>               |                              |
>   --------------------------------------------------->
>         |<--->|
>      margins.target
>         |->
>   vtime, done_vtime

All it does is shifting the vtime (and done_vtime) within the current window
so that we don't build up budget too lage a budget possibly spanning
multiple periods. The lagging detection is supposed to detect IOs which are
issued two+ periods ago which didn't finish in the last period. So, I don't
think the above sliding up the window affects that detection given that the
lagging detection is done before the window sliding. All it's checking is
whether there still are in-flight IOs which were issued two+ windows ago, so
how the last window has been fast forwarded shouldn't affect the detection,
no?

Thanks.

-- 
tejun