linux-kernel - Re: Re: [PATCH 03/15] sched/fair: Add lag based placement

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Message-ID: <00d22bc9-4abe-4e7d-9393-4025ed8c3642@bytedance.com>
Date:   Fri, 13 Oct 2023 16:14:55 +0800
From:   Abel Wu <wuyun.abel@...edance.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, yu.c.chen@...el.com,
        youssefesmat@...omium.org, joel@...lfernandes.org, efault@....de,
        tglx@...utronix.de
Subject: Re: Re: [PATCH 03/15] sched/fair: Add lag based placement

On 10/13/23 3:37 PM, Peter Zijlstra Wrote:
> On Thu, Oct 12, 2023 at 03:04:47PM +0800, Abel Wu wrote:
>> On 10/11/23 9:24 PM, Peter Zijlstra Wrote:
> 
>>>>> +		 * we should inflate the lag before placement such that the
>>>>> +		 * effective lag after placement comes out right.
>>>>> +		 *
>>>>> +		 * As such, invert the above relation for vl'_i to get the vl_i
>>>>> +		 * we need to use such that the lag after placement is the lag
>>>>> +		 * we computed before dequeue.
>>>>> +		 *
>>>>> +		 *   vl'_i = vl_i - w_i*vl_i / (W + w_i)
>>>>> +		 *         = ((W + w_i)*vl_i - w_i*vl_i) / (W + w_i)
>>>>> +		 *
>>>>> +		 *   (W + w_i)*vl'_i = (W + w_i)*vl_i - w_i*vl_i
>>>>> +		 *                   = W*vl_i
>>>>> +		 *
>>>>> +		 *   vl_i = (W + w_i)*vl'_i / W
>>>
>>> And then we obtain the scale factor: (W + w_i)/W, which is >1, right?
>>
>> Yeah, I see. But the scale factor is only for the to-be-placed entity.
>> Say there is an entity k on the tree:
>>
>> 	vl_k	= V - v_k
>>
>> adding the to-be-placed entity i affects this by:
>>
>> 	define delta := w_i*vl_i / (W + w_i)
>>
>> 	vl'_k	= V' - v_k
>> 		= V - delta - (V - vl_k)
>> 		= vl_k - delta
>>
>> hence for any entity on the tree, its lag is offsetted by @delta. So
>> I wonder if we should simply do offsetting rather than scaling.
> 
> I don't see the point, the result is the same and computing delta seems
> numerically less stable.

Right. I was not myself then, please forget what I said..