linux-kernel - RE: [PATCH v4] scsi: ufs: Fix imprecise load calculation in devfreq window

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Message-ID: <SN6PR04MB46405CE4B375BA3134D64A99FC800@SN6PR04MB4640.namprd04.prod.outlook.com>
Date:   Thu, 11 Jun 2020 08:03:35 +0000
From:   Avri Altman <Avri.Altman@....com>
To:     Stanley Chu <stanley.chu@...iatek.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "alim.akhtar@...sung.com" <alim.akhtar@...sung.com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "asutoshd@...eaurora.org" <asutoshd@...eaurora.org>
CC:     "beanhuo@...ron.com" <beanhuo@...ron.com>,
        "cang@...eaurora.org" <cang@...eaurora.org>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "bvanassche@....org" <bvanassche@....org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kuohong.wang@...iatek.com" <kuohong.wang@...iatek.com>,
        "peter.wang@...iatek.com" <peter.wang@...iatek.com>,
        "chun-hung.wu@...iatek.com" <chun-hung.wu@...iatek.com>,
        "andy.teng@...iatek.com" <andy.teng@...iatek.com>,
        "chaotian.jing@...iatek.com" <chaotian.jing@...iatek.com>,
        "cc.chou@...iatek.com" <cc.chou@...iatek.com>
Subject: RE: [PATCH v4] scsi: ufs: Fix imprecise load calculation in devfreq
 window


> 
> The UFS load calculation is based on "total_time" and "busy_time" in a
> devfreq window. However, the source of time is different for both
> parameters: "busy_time" is assigned from "jiffies" thus has different
> accuracy from "total_time" which is assigned from ktime_get().
> 
> Besides, the time of window boundary is not exactly the same as
> the starting busy time in this window if UFS is actually busy
> in the beginning of the window. A similar accuracy error may also
> happen for the end of busy time in current window.
> 
> To guarantee the precision of load calculation, we need to
> 
> 1. Align time accuracy of both devfreq_dev_status.total_time and
>    devfreq_dev_status.busy_time. For example, use "ktime_get()"
>    directly.
> 
> 2. Align below timelines,
>    - The beginning time of devfreq windows
>    - The beginning of busy time in a new window
>    - The end of busy time in the current window
> 
> Fixes: a3cd5ec55f6c ("scsi: ufs: add load based scaling of UFS gear")
> Signed-off-by: Stanley Chu <stanley.chu@...iatek.com>
Reviewed-by: Avri Altman <avri.altman@....com>

Just a small nit.

> -       stat->total_time = jiffies_to_usecs((long)jiffies -
> -                               (long)scaling->window_start_t);
> +       stat->total_time = ktime_to_us(curr_t) - scaling->window_start_t;
ktime_sub ?

Thanks,
Avri