linux-kernel - Re: [PATCH 06/10] sched/fair: Add avg

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Date:   Tue, 21 Mar 2023 21:58:13 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <mingo@...nel.org>, <vincent.guittot@...aro.org>,
        <linux-kernel@...r.kernel.org>, <juri.lelli@...hat.com>,
        <dietmar.eggemann@....com>, <rostedt@...dmis.org>,
        <bsegall@...gle.com>, <mgorman@...e.de>, <bristot@...hat.com>,
        <corbet@....net>, <qyousef@...alina.io>, <chris.hyser@...cle.com>,
        <patrick.bellasi@...bug.net>, <pjt@...gle.com>, <pavel@....cz>,
        <qperret@...gle.com>, <tim.c.chen@...ux.intel.com>,
        <joshdon@...gle.com>, <timj@....org>, <kprateek.nayak@....com>,
        <youssefesmat@...omium.org>, <joel@...lfernandes.org>
Subject: Re: [PATCH 06/10] sched/fair: Add avg_vruntime

On 2023-03-06 at 14:25:27 +0100, Peter Zijlstra wrote:
[...]
>  
> +/*
> + * Compute virtual time from the per-task service numbers:
> + *
> + * Fair schedulers conserve lag: \Sum lag_i = 0
> + *
> + * lag_i = S - s_i = w_i * (V - v_i)
> + *
The definination of above lag_i seems to be inconsistent with the defininatin
of se->lag in PATCH 8. Maybe rename lag_i to something other to avoid confusion?
> + * \Sum lag_i = 0 -> \Sum w_i * (V - v_i) = V * \Sum w_i - \Sum w_i * v_i = 0
> + *
> + * From which we solve V:
> + *
> + *     \Sum v_i * w_i
> + * V = --------------
> + *        \Sum w_i
> + *
> + * However, since v_i is u64, and the multiplcation could easily overflow
> + * transform it into a relative form that uses smaller quantities:
> + *
> + * Substitute: v_i == (v_i - v) + v
> + *
> + *     \Sum ((v_i - v) + v) * w_i   \Sum (v_i - v) * w_i
> + * V = -------------------------- = -------------------- + v
> + *              \Sum w_i                   \Sum w_i
> + *
> + *
Not sure if I understand it correctly, does it mean  (v_i - v) * w_i will not
overflow? If the weight of task is 15 (nice 19), then if v_i - v > (S64_MAX / 15)
it gets overflow. Is it possible that v_i is much larger than cfs_rq->min_vruntime
in this case?

thanks,
Chenyu